Izu-Bonin-Mariana Arc - Geologic History of The IBM Arc System

Geologic History of The IBM Arc System

The evolution of the IBM arc system is among the best known of any convergent margin. Because IBM has always been an arc system under strong extension, its components encompass a broad area, from the Palau-Kyushu Ridge to the IBM trench (see first-right figure). In general, the oldest components are farthest west, but a complete record of evolution is preserved in the forearc. The IBM subduction zone began as part of a hemispheric-scale foundering of old, dense lithosphere in the Western Pacific (Stern & Bloomer 1992). The beginning of true subduction localized the magmatic arc close to its present position, about 200 km away from the trench, and allowed the sub-forearc mantle to stabilize and cool. The arc stabilized until about 30 Ma, when it began to rift to form the Parece Vela Basin. Spreading also began in the northernmost part of the IBM arc about 25 Ma and propagated south to form the Shikoku Basin. Parece Vela and Shikoku basin spreading systems met about 20 Ma and the combined Parece Vela Basin-Shikioku Basin continued widening until about 15 Ma, ultimately producing Earth's largest back-arc basin. The arc was disrupted during rifting but began to build again as a distinct magmatic system once seafloor spreading began. Arc volcanism, especially explosive volcanism, waned during much of this episode, with a resurgence beginning about 20 Ma in the south and about 17 Ma in the north. Tephra from northern and southern IBM show that strong compositional differences observed for the modern arc have existed over most of the arc's history, with northern IBM being more depleted and southern IBM being relatively enriched. About 15 Ma, the northernmost IBM began to collide with Honshū, probably as a result of new subduction along the Nankai Trough. A new episode of rifting to form the Mariana Trough back-arc basin began sometime after 10 Ma, with seafloor spreading beginning about 3-4 Ma. Because disruption of the arc is the first stage in forming any back-arc basin, the present Mariana arc volcanoes cannot be older than 3-4 Ma but the Izu-Bonin volcanoes could be as old as ~25 Ma. The Izu interarc rifts began to form about 2 Ma.