It has been suggested that the Shatsky Rise oceanic plateau formation began simultaneously with a reorganization of spreading at a triple junction bordering the northern Pacific plate, and this coincidence has led to speculation about the connections between the two events. We present new marine geophysical data that constrain the seafloor spreading history of the Pacific-Izanagi-Farallon triple junction just before the birth of the Shatsky Rise. Bathymetric data reveal en echelon, abandoned spreading centers trending northwest-southeast located adjacent to the southwest flank of the Shatsky Rise. Magnetic anomalies and bathymetry are interpreted to indicate that segments of the Pacific-Farallon Ridge near the triple junction propagated northwest from chron M23 (153 Ma) to chron M22 (151 Ma) during a spreading ridge reorganization at the edge of a likely microplate. Our detailed examination of bathymetric and magnetic anomaly lineations also shows that the strike of the Pacific-Izanagi Ridge changed gradually on the west side of the triple junction around chron M22. Our observations indicate that the plate boundary reorganization began several million years before the formation of the Shatsky Rise, implying that the eruption of the plateau did not cause the reorganization.

Current interpretations of Cretaceous tectonic evolution of the northwest Pacific trace interactions between the Pacific plate and three other plates, the Farallon, Izanagi, and Kula plates. The Farallon plate moved generally eastward relative to the Pacific plate. The Izanagi and Kula plates moved generally northward relative to the Pacific plate, with Izanagi the name given to the northward-moving plate prior to the Cretaceous normal polarity superchron and the name Kula applied to the postsuperchron plate. In this article I suggest that these names apply to the same plate and that there was only one plate moving northward throughout the Cretaceous. I suggest that the tectonic reorganization that has previously been interpreted as formation of a new plate, the Kula plate, at the end of the superchron was actually a plate boundary reorganization that involved a 2000 km jump of the Pacific–Farallon–Kula/Izanagi triple junction. Because this jump occurred during a time of no magnetic reversals, it is not possible to map or date it precisely, but evidence suggests mid-Cretaceous timing. The Emperor Trough formed as a transform fault linking the locations of the triple junction before and after the jump. The triple junction jump can be compared with an earlier jump of the triple junction of 800 km that has been accurately mapped because it occurred during the Late Jurassic formation of the Mesozoic-sequence magnetic lineations. The northwest Pacific also contains several volcanic features, such as Hawaii, that display every characteristic of a hotspot, although whether deep mantle plumes are a necessary component of hotspot volcanism is debatable. Hawaiian volcanism today is apparently independent of plate tectonics, i.e., Hawaii is a center of anomalous volcanism not tied to any plate boundary processes. The oldest seamounts preserved in the Hawaii-Emperor chain are located on Obruchev Rise at the north end of the Emperor chain, close to the junction of the Aleutian and Kamchatka trenches. These seamounts formed in the mid-Cretaceous close to the spreading ridge abandoned by the 2000 km triple junction jump. Assuming that Obruchev Rise is the oldest volcanic edifice of the Hawaiian hotspot and thus the site of its initiation, the spatial and temporal coincidence between these events suggests that the Hawaii hotspot initiated at the spreading ridge that was abandoned by the 2000 km jump of the triple junction. This implies a tectonic origin for the hotspot. Other volcanic features in the northwest Pacific also appear to have tectonic origins. Shatsky Rise is known to have formed on the migrating Pacific-Farallon-Izanagi triple junction during the Late Jurassic–Early Cretaceous, not necessarily involving a plume-derived hotspot. Models for the formation of Hess Rise have included hotspot track and anomalous spreading ridge volcanism. The latter model is favored in this article, with Hess Rise forming on a ridge axis possibly abandoned as a result of a ridge jump during the superchron. Thus, although a hotspot like Hawaii could be associated with a deep mantle plume today, it would appear that it and other northwest Pacific volcanic features originally formed as consequences of shallow plate tectonic processes.

Shatsky Rise is an oceanic plateau that formed at the Pacific-Farallon-Izanagi triple junction during the Late Jurassic to Early Cretaceous. Its origin is unclear, but volcanism from a mantle plume or plume head is accepted as an explanation because many observations from the plateau are consistent with the plume head model. Initial eruptions were massive and rapid, with emplacement rates estimated at 1.2–4.6 km 3 /yr, similar to continental flood basalts. The plateau exhibits an age progression, with igneous output waning over time, possibly representing the transition from plume head to plume tail. Shallow water fossils imply that the rise top was subaerial and that thermal and dynamic uplift was significant. Furthermore, the age progression and trends of Shatsky and Hess rises are mimicked by the Mid-Pacific Mountains, to be expected if these features formed by the drift of the plate over mantle plumes. In contrast, several observations do not fit the plume head model. The initial eruption was coincident with a reorientation of the Pacific-Izanagi ridge and an 800-km jump of the triple junction, a low probability occurrence if plume heads behave independently of plate motions. Moreover, this same type of event may have occurred repeatedly, as other western Pacific plateaus occur near the trace of this triple junction (Hess Rise) and the Pacific-Farallon-Phoenix triple junction (Magellan Rise, Manihiki Plateau, Mid-Pacific Mountains). If these other plateaus formed from plumes, either there were many plumes or the plumes defining the triple junction paths exhibited large relative motion. Moreover, rocks recovered from the main Shatsky Rise edifices have mid-ocean ridge basalt (MORB) geochemistry and isotopic signatures, whereas most plume head models imply that lower mantle material, with a different signature, will be carried to the surface. A simpler hypothesis is that Shatsky Rise and other near-triple-junction plateaus were formed as a result of ridge tectonics. One possibility is that decompression melting occurred near the triple junctions during the Mesozoic because the northwest Pacific was located over a region of anomalous asthenosphere that was susceptible to melting given only small perturbations in lithospheric stress. A pitfall for this argument is that changes in the thin, fast-spreading Pacific lithosphere must result in massive volcanism. Although both plume and ridge tectonics hypotheses explain some observations from Shatsky Rise, uncertainties make it premature to conclude which, if either, is correct. Resolution awaits future investigations, which must reveal missing pieces to this puzzle.

Reconstruction of the tectonic evolution of the Pacific basin indicates a direct relationship between intraplate volcanism and plate reorganizations, which suggests that volcanism was controlled by fracturing and extension of the lithosphere. Middle Jurassic to Early Cretaceous intraplate volcanism included oceanic plateau formation at triple junctions (Shatsky Rise, the western Mid-Pacific Mountains) and a diffuse pattern of ocean island volcanism (Marcus Wake, Magellan seamounts) reflecting an absence of any well-defined stress field within the Pacific plate. The stress field changed in the Early Cretaceous when accretion of the Insular terrane to the North American Cordillera and the Median Tectonic arc to New Zealand stalled migration of the Pacific-Farallon and Pacific-Phoenix ocean ridges, leading to the generation of the Ontong Java, Manahiki, Hikurangi, and Hess Rise oceanic plateaus. Plate reorganizations in the Late Cretaceous resulted from the breakup of the Phoenix and Izanagi plates through collision of the Pacific-Phoenix ocean ridge with the southwest margin of the basin and development of island arc–marginal basin systems in the northwestern part of the basin. The Pacific plate nonetheless remained largely bounded by spreading centers, and intraplate volcanism followed preexisting lines of weakness in the plate fabric (Line Islands) or resulted from fractures generated by ocean ridge subduction beneath island arc systems (Emperor chain). The Pacific plate began to subduct under Asia in the Early Eocene as inferred from the record of accreted material along the Japanese margin. Further changes to the stress field at this time resulted from abandonment of the Kula-Pacific and the North New Guinea (Phoenix)–Pacific ridges and from development of the Kamchatkan and Izu-Bonin-Mariana arcs, leading to the generation of the Hawaiian chain as a propagating fracture. The final major change in the stress field occurred in the Late Oligocene as a result of breakup of the Farallon into the Cocos and Nazca plates, which caused a hiatus in Hawaiian volcanism; initiated the Sala y Gomez, Foundation, and Samoan chains; and terminated the Louisville chain. The correlations with tectonic events are compatible with shallow-source models for the origin of intraplate volcanism and suggest that the three principal categories of volcanism, intraplate, arc, and ocean ridge, all arise from plate tectonic processes, unlike in plume models, where intraplate volcanism is superimposed on plate tectonics.