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Figure F6. Volcano-tectonic domains within Izu arc. The well-defined arc (volcanic) front is formed of the largest volcanoes, including island volcanoes, the largest are named here. Arc crust underlays the region behind the arc front (i.e., west of the arc, farther from the trench), the rear arc, which is bounded to the west by the Shikoku Basin, which is floored by oceanic crust. The rear arc is divided into tectonic zones, from west to east (also oldest to youngest): (1) rear-arc seamount chains (~100 km long; ~17–3 Ma) span basalt to rhyolite compositional range (Fig. F12A); (2) extensional zone (~100 km wide; <3 Ma) overlaps the eastern half of the rear-arc seamount chains and is characterized by ~north–south normal faults with small bimodal volcanoes (back-arc knolls); and (3) active rift, a narrower extensional zone that consists of a series of north–south rift basins immediately behind the arc front, also with bimodal volcanoes. Magmatism in the active rifts and extensional zone is referred to as rift-type magmas and in the rear-arc seamount chains as rear-arc seamount chain–type magmas. White stars = Site U1436 (fore arc) and Site U1437 (rear arc). Box = area of Figure F22.

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