Structural evolution of preexisting oceanic crust through intraplate igneous activities in the Marcus‐Wake seamount chain

<jats:p>Multichannel seismic reflection studies and seismic refraction surveys with ocean bottom seismographs in the Marcus‐Wake seamount chain in the northwestern Pacific Ocean reveal <jats:italic>P</jats:italic> wave velocity structures of hot spot‐origin seamounts and adjacent oceanic crust. Inside the seamounts are central high‐velocity (>6.5 km/s) structures extending nearly to the top that may indicate intrusive cores. Thick sediment layers (up to 4 km) with <jats:italic>P</jats:italic> wave velocities of 4–5 km/s have accumulated on seafloor that predates seamount formation. Downward crustal thickening of up to 2 km was documented beneath a large seamount cluster, but thickening was not confirmed below a small seamount cluster. Volume ratios of an intrusive core to a seamount body are 15–20%, indicating that most of the supplied magma was consumed in forming the thick sedimentary and volcaniclastic layer constituting the seamount flanks. Underplating and downward crustal thickening may tend to occur when second or later intrusive cores are formed in a seamount. <jats:italic>P</jats:italic> wave velocities in the lowest crust and in the uppermost mantle below the seamount chain are 0.1–0.2 km/s higher and 0.3–0.5 km/s lower, respectively, than velocities below oceanic crust. We explain this difference as a result of sill‐like intrusion of magma into the lower crust and uppermost mantle. Reflected waves observed at offsets >200 km are from mantle reflectors at depths of 30–45 km and 55–70 km. The shallower reflectors may indicate structures formed by intraplate igneous activities, and the deeper reflectors may correspond to the lithosphere‐asthenosphere boundary.</jats:p>