Figure F5. Temporal variation of locations of volcanism in the Izu-Bonin arc (left) and location vs. 40Ar/39Ar age plot for the rear-arc volcanics (upper right) (after Ishizuka et al., 2003b). Ocean Drilling Program Sites 786 and 792 were drilled ~1000 m through volcaniclastic sediment into Eocene basement. Red dots in <1–3 Ma boxes = samples with “rift-type” compositions; red dots in 3–17 Ma boxes = samples with “rear-arc type” compositions (shown in Fig. F6). Dating of dredged lavas provides only a minimum age for the edifice and cannot constrain relative eruption rates, which can be estimated from sedimentation rates. The data from Enpo chain (red outline in right map) just south of the proposed drill site are shown in the geochemical figures with other rear-arc chains to show the essential differences between the volcanic front, rift-type, and rear-arc type magma compositions. Yellow stars = proposed drill sites. L1–L5 = seismic layers summarized in Table T1 and described in detail in text.
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