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This paper focuses on the processes of arc rifting in the context of the volcanic, structural, and sedimentologic evolution of the Izu-Bonin-Mariana arc-trench system. Middle and late Eocene supra-subduction zone magmatism formed a vast terrane of boninites and island arc tholeiites (>300 × 3000 km) that is unlike active arc systems but is similar to many ophiolites. A modern-style volcanic arc developed by the Oligocene, along which intense tholeiitic and calc-alkaline volcanism continued until 29 (Mariana) and 27 Ma (Izu-Bonin). The Eocene-Oligocene arc massif was stretched during protracted Oligocene rifting, creating sags and half graben in the forearc and backarc. Minima in arc volcanism (29-27 Ma in the Marianas and 23-20 Ma in the Izu-Bonins) occurred during early phases of backarc spreading in the Parece Vela and Shikoku basins, respectively. Western Shikoku Basin oceanic crust previously identified as formed during Subchrons 6C (25 Ma) to 6A, may instead have formed during Subchrons 6A (22 Ma) to 5D, with a spreading jump over the eastern 5D and 5E magnetic lineations, leaving only 6A lineations east of the axial 5B-5C lineations. Middle Miocene to Holocene Izu-Bonin volcanism developed a volcanic front oriented 3° counterclockwise from the Oligocene frontal arc and has increased in intensity to a Pliocene-Quaternary maximum. Neogene magmatism along the volcanic front has been focused on bimodally spaced (27 and 47 km), long-lived centers, but arc tholeiites have occasionally intruded the thermally cool forearc, and cross-chains of mostly submarine volcanoes occur in the backarc immediately west of the frontal arc volcanoes as well as further west, on the edge of the rifted Oligocene arc crust. The present rifting of the central Izu-Bonin arc began about 2 Ma. At 30° 55'N, in Sumisu Rift, 1.1 ± 0.4 km of subsidence of the inner rift basement and 1.1 ± 0.5 km of uplift of the rift flank are associated with the 2-2.5 km throw on the eastern border fault zone. A zigzag pattern of half-graben-bounding normal faults characterizes both the Oligocene and Quaternary rifts. Except for the greater extent (Basin and Range style) of the Oligocene structures, the two fault patterns appear similar and little-influenced by pre-existing structures. Many of the border faults and rift flank uplifts developed early in the rift history. Syn-rift sediments are pervasively faulted and often intruded. Both the Sumisu Rift at Sites 790 and 791 and the forearc basin at Site 793 are floored with syn-rift volcanics that are geochemically distinct from their contemporary frontal arc volcanics. The oldest (> 1.1 Ma) to the youngest (Holocene) Sumisu Rift lavas are backarc basin basalts, whereas preand syn-rift arc volcanism is mostly low-K, subalkaline, rhyolite and andesite. The Oligocene forearc volcanics are dominantly high-Mg, low-Ti, two-pyroxene, basaltic andesites and andesites, similar to the Eocene volcanics of the outer arc high. Coarse volcanogenic sediments, derived from contemporaneous frontal arc volcanism, dominate both the Sumisu Rift and the Oligocene forearc basin fill. They were rapidly (>250 m/m.y.) deposited by turbidity currents and debris flows onto rift-floor sediment plains. Sedimentation patterns were directly influenced by the productivity of the proximal arc volcanoes, with volcanic lulls recorded by hemipelagic interbeds. Many arc segments go through a cycle of (1) frequent volcanism before and during rifting; (2) reduced and/or less disseminated volcanism during latest rifting and early backarc spreading, as new frontal arc volcanoes are being constructed and growing to sea level; and (3) increasingly vigorous volcanism during middle and late stage backarc spreading, and until the next rift cycle begins. Even within periods of intense volcanism, 100-km-long arc segments may be quiescent for periods of up to 400 k.y. The frontal arc volcanoes, because of their thicker crust and higher heat flow, create a linear zone of lithospheric weakness that controls the location of arc rifting. Differences in plate boundary forces at the ends, more than in the middle, of volcanic arcs may significantly influence their proclivity to rift.

Rifting and the Volcanic-Tectonic Evolution of the Izu-Bonin-Mariana Arc