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

Abstract

[1] Multichannel seismic reflection studies and seismic refraction surveys with ocean bottom seismographs in the Marcus-Wake seamount chain in the northwestern Pacific Ocean reveal P 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 P 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. P 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.

Wide-angle refraction experiments in the Daito Ridges region at the northwestern end of the Philippine Sea plate

“…In addition, most seismic refraction lines performed in previous studies did not reach the required offsets. Kaneda et al (2010) found deeper reflectors at a depth of 35 to 45 km beneath seamounts with a hot spot origin in the northwestern Pacific Basin. They inferred that these reflectors indicate structures that formed via intraplate igneous activities, such as at the top of a mantle partial melting zone or a remnant of a hot spot plume head.…”

Petrological interpretation of deep crustal intrusive bodies beneath oceanic hotspot provinces

“…A recent seismic survey was conducted along the Marcus‐Wake seamount chain [ Kaneda et al ., ], where its associated volcanic edifice was emplaced on a super plume environment in the northwest Pacific basin. Their main results are high‐velocity, intrusive cores at shallow depths and low upper mantle velocities (7.5–8.0 km/s).…”

Active-source seismic survey on the northeastern Hawaiian Arch: insights into crustal structure and mantle reflectors

“…One interpretation is that the heterogeneities are remnants of intraplate igneous activities such as the top of a zone of partial mantle melting or part of a hotspot plume head . Kaneda et al (2010) observed reflected waves from depths of 30-45 km beneath the Marcus-Wake seamount chain, and they interpreted the reflectors to have originated from intraplate igneous activity. However, we detected mantle reflectors beneath the normal ocean basin, where crustal thickening due to intraplate volcanism does not occur.…”

“…Because it is hard to constrain Vp in the mantle, which Pn waves do not penetrate, we extrapolated Vp to the uppermost mantle following the method of Kaneda et al (2010) and Ohira et al (2017b) (Fig. 13).…”