Imaging the seismic lithosphere-asthenosphere boundary of the oceanic plate

1 The seismic lithosphere-asthenosphere boundary (LAB) or G discontinuity, a seismologically characterized abrupt drop in wave speed in the uppermost mantle, is one of the key issues in current geodynamics. Although plate tectonics started as a theory for the ocean, reports on LAB for normal oceanic regions are scarce due to paucity of seismic data, and whether or not the oceanic LAB grows with age is the key issue to be resolved. We conduct a systematic survey for the oceanic LAB using S-to-p converted seismic waves along three margins of oceanic plates whose crustal age ranges from ∼10 Myr to ∼130 Myr, and we observe laterally continuous oceanic LAB images. The thickness of the oceanic plate estimated from LABs increases with the plate age, though scattered, suggesting that the evolution of oceanic lithosphere is predominantly governed by temperature and that the oceanic seismic LAB represents a boundary that grows with age.