The Shatsky Rise is one of the largest oceanic plateaus, a class of volcanic features whose formation is poorly understood. It is also a plateau that was formed near spreading ridges, but the connection between the two features is unclear. The geologic structure of the Shatsky Rise can help us understand its formation. Deeply penetrating two-dimensional (2-D) multichannel seismic (MCS) reflection profiles were acquired over the southern half of the Shatsky Rise, and these data allow us to image its upper crustal structure with unprecedented detail. Synthetic seismograms constructed from core and log data from scientific drilling sites crossed by the MCS lines establish the seismic response to the geology. High-amplitude basement reflections result from the transition between sediment and underlying igneous rock. Intrabasement reflections are caused by alternations of lava flow packages with differing properties and by thick interflow sediment layers. MCS profiles show that two of the volcanic massifs within the Shatsky Rise are immense central volcanoes. The Tamu Massif, the largest (~450 km × 650 km) and oldest (ca. 145 Ma) volcano, is a single central volcano with a rounded shape and shallow flank slopes (<0.5°–1.5°), characterized by lava flows emanating from the volcano center and extending hundreds of kilometers down smooth, shallow flanks to the surrounding seafloor. The Ori Massif is a large volcano that is similar to, but smaller than, the Tamu Massif. The morphology of the massifs implies formation by extensive and far-ranging lava flows emplaced at small slope angles. The relatively smooth flanks of the massifs imply that the volcanoes were not greatly affected by rifting due to spreading ridge tectonics. Deep intrabasement reflectors parallel to the upper basement surface imply long-term isostasy with the balanced addition of material to the surface and subsurface. No evidence of subaerial erosion is found at the summits of the massifs, suggesting that they were never highly emergent.

The eruptive history of the Shatsky Rise, a large oceanic plateau in the northwestern Pacific Ocean, is poorly understood. Although it has been concluded that the Shatsky Rise volcanic edifices erupted rapidly, there are few solid chronological data to support this conclusion. Similarly, the Shatsky Rise is thought to have formed near the equator, but paleolatitude data from the plateau are few, making it difficult to assess its plate tectonic drift with time. To understand the formation history of this oceanic plateau, paleomagnetic measurements were conducted on a total of 362 basaltic lava samples cored from the Shatsky Rise at 4 sites (U1346, U1347, U1349, and U1350) during Integrated Ocean Drilling Program Expedition 324. Examining changes in paleomagnetic inclinations, we gain a better understanding of eruptive rates by comparison of observed shifts in inclination with expected paleosecular variation. At three sites (U1346, U1347, and U1349) little change in paleomagnetic directions was observed, implying that the cored sections were mostly erupted rapidly over periods of <~100–200 yr. Only Site U1350 displayed directional changes consistent with significant paleosecular variation, implying emplacement over a period of ~1000 yr. The paleomagnetic data are consistent with the idea that the Shatsky Rise igneous sections were mostly emplaced rapidly, but there were some time gaps and some fl ank locations built up more slowly. Because paleosecular variation was inadequately sampled at all the Shatsky Rise sites, paleolatitudes have large uncertainties, and because of the equatorial location, magnetic polarity is also uncertain. All sites yield low paleolatitudes and indicate that the Shatsky Rise stayed near the equator during its formation. Given that the locus of magmatism moved northward relative to the Pacific plate while staying near the equator, the Pacific plate must have drifted southward relative to the spin axis during the emplacement of the plateau.