Structural variation of the oceanic Moho in the Pacific plate revealed by active-source seismic data

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

“…For example, a seismic refraction survey in a section of the northwest Pacific basin, which was produced at a half-spreading rate of > 80 mm/year, observed a mantle Vp of 8.5-8.7 and 7.9 km/s along lines parallel and perpendicular to the magnetic lineation, respectively; these values indicate an azimuthal anisotropy of 7-10% in the uppermost mantle (Oikawa et al 2010). However, another area of the Pacific basin that was created at an intermediate spreading rate (< 40 mm/year) also showed high Vp of up to 8.6 km/s in the uppermost mantle parallel to the direction of seafloor spreading, although the degree of azimuthal anisotropy was not constrained (Ohira et al 2017a). Therefore, the azimuthal anisotropy of ~ 9% observed in this study suggests that the strong anisotropy in the uppermost mantle is explained by not only a spreading rate but also some other factor.…”

“…e-h Depth-converted seismic reflection images of the images in panels a-d; Vp structures are superimposed on each reflection image. The black lines show Vp at the e 205 km and f 575 km points on the EW line and at g the 198 km point on the NS1 line and h the 75 km point on the NS2 line be generated by a crust-mantle transition layer in which Vp gradually increases from the lowermost crust to the uppermost mantle (Ohira et al 2017a). The high-amplitude Pt arrivals generally start to appear at an offset of ~ 20 km on all records.…”

Electrical conductivity of old oceanic mantle in the northwestern Pacific I: 1-D profiles suggesting differences in thermal structure not predictable from a plate cooling model

“…The difference in the conductivity of the uppermost Shinohara et al (2008) and Ohira et al (2017), respectively…”

“…Because carbon dioxide-rich melt is expected from the highly vesicular petit-spot basalt samples (Okumura and Hirano 2013), the mantle beneath the petit-spot field may be relatively enriched in carbon, which may be one of the causes of the difference in the electrical structure of Area D from that of Area C. For Area A and Area B, the plume associated with the formation of the Shatsky Rise may have affected the initial state of the temperature and/or composition of the mantle, although the two areas are located outside of the topographic anomaly of the Shatsky Rise itself. Ohira et al (2017) found that along the spreading direction southeast of Area B, there are areas where the Moho is diffuse, weak, or absent, and are thus characterized by the presence of a gradual crust-mantle transition in seismic velocity. These authors suggested that the formation time of the crust-mantle transition layer is coincident with that of the Shatsky Rise and inferred a causal relationship.…”

“…9). Shinohara et al (2008) conducted a seismic survey in the eastern end of our array in Area A and showed that the sediment layer is as thick as ~400 m. The seismic survey by Ohira et al (2017) along a line 150 km southeast of our array in Area B showed that the thickness of the sediment layer is ~300 m. Both studies reported similar characters; the P-wave velocity structure in the crust below the sediment indicated typical oceanic crust, except for the transitional layer from the crust to the mantle. Although our models do not resolve the shallowest layer, which is thinner than 1 km, the thin but very conductive sediment layer can influence the mean conductivity in the uppermost layer in the inversion.…”