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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.

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