Seismic structures of the 154-160 Ma oceanic crust and uppermost mantle in the Northwest Pacific Basin

We present detailed P-wave velocity models for the Northwest Pacific Basin which were produced in 154-160 Ma at a high seafloor spreading half-rate of >8 cm/yr and have not been appreciably deformed by tectonic or igneous activity since then. We carried out wide-angle seismic experiments on two crossing survey lines which are respectively parallel and perpendicular to paleomagnetic lineations. The seismic crustal models for both lines are almost identical and homogeneous along these lines. The crust consists of an upper layer (Layer 2) with a P-wave velocity V-p = 2.5-6.8 km/s and a thickness of 1.3-2.2 km, and a lower layer (Layer 3) with a velocity of 6.8-7.1 km/s and a thickness of 4.6-5.9 km. These characteristics indicate that the crust beneath the survey line has a standard oceanic crustal structure. The structure of the uppermost mantle of the line parallel to the seafloor spreading direction exhibits considerable V-p heterogeneity within 5 km immediately below the Moho and shows an unusually high V-p of 8.5-8.7 km/s. The P-n velocity for the perpendicular line is 7.9 km/s, and the magnitude of the velocity anisotropy for the uppermost mantle amounts to a large value of 7-10%.