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

Abstract

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%.

Backarc basin oceanic crust and uppermost mantle seismic velocity structure of the Shikoku Basin, south of Japan

“…1). The direction of the seismic lines is roughly east-west, parallel to the direction of the backarc basin spreading, with a half-rate of 2.3-4.7 cm/year in 27-20 Ma except for the last NE-SW spreading stage, which occurred at a slower rate of 2-3 cm/year in 20-15 Ma (Okino et al, 1999). A tuned array of 36 airguns with a total volume of 8,040 inch 3 (132 l) was used as a controlled seismic source and shot at an interval of 200 m (90 s) for the wide-angle seismic measurements and at 50 m for the MCS (480 channels, 60 fold) measurements.…”

“…We recently obtained a detailed V p model for a normal oceanic crust in the Northwest Pacific Basin (NPB) using a modern seismic exploration method (Oikawa et al, 2010). Our results revealed that the oceanic crust in this basin was predictably homogeneous but that the uppermost mantle was unexpectedly heterogeneous with large azimuthal anisotropy.…”

“…In 2005 and 2006, we carried out three longoffset wide-angle seismic and multi-channel seismic reflection (MCS) experiments across the SB using a large airgun array and a large number of receivers. The data were analyzed using a combination of forward modeling and tomographic inversion techniques identical to those used by Oikawa et al (2010). In this paper, we present detailed Copyright c The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB.…”

“…The SB and PVB were produced at 30-15 Ma at moderate half-spreading rates of 2.0-4.7 cm/year, and the Kinan Seamount Chain lies close to the extinct spreading center of the SB (e.g. Kobayashi et al, 1995;Okino et al, 1999). In the 1960s, P-wave velocity (V p ) models of these marginal basins in the Philippine Sea were obtained by two-ship refraction experiments (e.g., P3 and P9, shown in Fig.…”

Executive Summary: “Mantle Frontier” Workshop

“…Seismic images of the Moho can vary from sharp to diffuse boundaries, which may correspond to the geologic variety found at the crust/ mantle transition in ophiolites. Strong seismic azimuthal anisotropy can be expected to start immediately beneath the Moho, such as measured in the NW Pacific (Oikawa et al, 2010). Lower crustal dipping reflectors matching fast Vp directions may be manifestations of basal shear near the Moho.…”