Shuichi Kodaira

Evidence for frozen melts in the mid-lithosphere detected from active-source seismic data

The interactions of the lithospheric plates that form the Earth’s outer shell provide much of the evidentiary basis for modern plate tectonic theory. Seismic discontinuities in the lithosphere arising from mantle convection and plate motion provide constraints on the physical and chemical properties of the mantle that contribute to the processes of formation and evolution of tectonic plates. Seismological studies during the past two decades have detected seismic discontinuities within the oceanic lithosphere in addition to that at the lithosphere–asthenosphere boundary (LAB). However, the depth, distribution, and physical properties of these discontinuities are not well constrained, which makes it difficult to use seismological data to examine their origin. Here we present new active-source seismic data acquired along a 1,130u2009km profile across an old Pacific plate (148–128u2009Ma) that show oceanic mid-lithosphere discontinuities (oceanic MLDs) distributed 37–59u2009km below the seafloor. The presence of the oceanic MLDs suggests that frozen melts that accumulated at past LABs have been preserved as low-velocity layers within the current mature lithosphere. These observations show that long-offset, high-frequency, active-source seismic data can be used to image mid-lithospheric structure, which is fundamental to understanding the formation and evolution of tectonic plates.

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

Seismic refraction and multi-channel seismic reflection surveys were conducted on the northeastern Hawaiian Arch to examine the effect of hotspot volcanism on the seismic structure of the crust and uppermost mantle. The crustal thickness deduced from the refraction data was typical for oceanic crust, which suggests that magmatic underplating does not occur, at least in our survey area, although the crustal seismic velocity may be influenced by flexure of the lithosphere on the arch. We identified high P-wave velocities (~u20098.65xa0km/s) in the uppermost mantle parallel to the paleo-seafloor spreading direction, which indicates that the shallower mantle structure immediately below the Moho preserves the original structure formed at a mid-ocean ridge. Moreover, we observed wide-angle reflection waves at large offsets in ocean-bottom seismometer records. The travel time analysis results showed that these waves were reflected from mantle reflectors at depths of 30–85xa0km below the seafloor, which are considered to represent heterogeneities consisting of frozen melts created during the cooling of the plate.