ABSTRACT

In order to estimate the active period of the Ayu Trough, which lies at the boundary between the Philippine Sea Plate and the Caroline Plate, we measured K-Ar ages of submarine rocks dredged from the region. Samples recovered from the trough axis are similar to typical MORB and/or BABB in chemical and petrological features and show K-Ar ages of around 1 Ma. This suggests that the Ayu Trough might have been a spreading center at least during the last 1 million years. While, samples recovered at a site 62 km east of the axial rift valley show K-Ar ages of 20–25 Ma, and they are high-alkali andesites. If these samples indicate that an island arc existed there at 25 Ma, the activity of the Ayu Trough might have started at 25 Ma or later.