Characteristics of Moho transition zone: MCS reflection records and petrological aspects and physical properties
Abstract
The Moho is defined as the seismological discontinuity at the crust and mantle boundary. Its global depth, thickness of transition zone, and velocity structure has not been studied well. It is also poorly known whether the Moho has the same petrological and seismological properties in the continent and in the ocean, or not. Previous studies propose several petrological models for the Moho: 1) phase transition boundary from basalt to eclogite, and 2) material boundary of mafic and ultramafic rocks. By the petrological observation in the Oman ophiolite, the oceanic crust is modeled as 3) diabase-homogeneous gabbro - layered gabbro - Moho transition layer - harzburgite. The thickness of Moho transition zone (MTZ), at the boundary between Earth's crust and the subjacent mantle, has significant effect on the seismic responses from the Moho. We examined seismic characteristics of Moho reflection (hereafter PmP) using MCS (Multi Channel Seismic) reflection records obtained by high quality seismic experiments in the western Pacific by JOGMEC (Japan Oil, Gas and MEtals national Corporation). The MCS records show clear reflections at ~ 6-10 km in depth from the ocean bottom in the north and south of Ogasawara Plateau. However, considering horizontal variation in the PmP intensity, the nature of the MTZ varies from place to place. In the seismic profile D00-D, across Ogasawara Plateau in the N-S direction, the PmP abruptly disappears far from the nearby seamount where the overlain sedimentary section has less change. In another case, shown in D00-C that is located 130km west of D00-D, the PmP clearly shows high-amplitude continuous reflection near the seamount's flank. Data acquisition is relatively constant for the Ogasawara MCS reflection lines; therefore, the difference in the PmP intensity between D00- D and D00-C may relate to the nature of the Moho. The comparison of reflection records and synthetic waveforms calculated by Tsuruga et al.(this meeting) shows that if the gradient of the Moho transition zone is greater than 1 (km/s)/km, PmP can be observed by the current MCS survey equipments. If the dominant frequency of the MCS reflection survey is ~15 Hz, penetrating down to the Moho depth, then the thickness of the Moho to identify the PmP should be less than a few hundred meters. The MCS reflection records in the western Pacific and the western Philippine Sea Basin suggest that the thickness of MTZ varies from ~100 m to over a few kilometers. This is consistent with the petrological observation in Oman ophiolite, sections of oceanic crust and possible mantle rock, showing the thickness of the mafic crust to ultra-mafic mantle transition varies from the order of meters to a few kilometers.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.S11C1773K
- Keywords:
-
- 3025 Marine seismics (0935;
- 7294);
- 7205 Continental crust (1219);
- 7208 Mantle (1212;
- 1213;
- 8124);
- 7218 Lithosphere (1236);
- 7220 Oceanic crust