Bonin Arc-Trench System

Abstract

Leg 126Preliminary Report page 7 ABSTRACTThe Izu-Bonin intra-oceanic island arc is a product of the sübduction of Pacific lithosphere since the Eocene. The focus of Ocean Drilling Program (ODP) Leg 126 was to study three important and poorly understood aspects of this system, namely (1) the origin and evolution of the forearc, investigated by drilling a series of holes through the sediments and into the basement of the forearc basin (Sites 787 and 792-793); (2) the process and products of arc rifting, investigated by drilling holes into the center (Sites 790/791) and eastern footwall (Sites 788/789) of the Sumisu backarc rift; and (3) recycling of subducted lithosphere and evolution of the mantle, investigated indirectly from the composition of the volcanic rocks recovered from all sites.The principal drilling results of Leg 126 show that the forearc basin formed between 31 and 34 Ma by separation of the formerly contiguous frontal and outer arc highs. Igneous basement beneath the center of the forearc basin includes high-Mg series andesites, andesites with boninitic affinities, and low-Mg series lavas with tholeiitic affinities. Following a minimum of volcanic output between 24 and 13 Ma, there has been a steady increase in explosive volcanic activity in the forearc with a dramatic increase in the late Quaternary. Paleomagnetic evidence shows that the forearc has been translated about 15°N since 30 Ma. Benthic foraminifer^ data suggest 1-2 km of basement uplift has occurred since the middle Oligocene. Pore waters in the Oligocene volcanogenic sediments of the forearc basin are the most extensively altered pore water of sea-water origin ever sampled by DSDP/ODP. Low-temperature alteration of the volcanogenic sediments has produced fluids extremely enriched in calcium and depleted in magnesium, silica, and sulfate.The present stage of rifting in the Sumisu Rift at 31°N began between 3.56 and 1.1 Ma with both present-day and pre-rift volcanism along the volcanic front dominated by rhyolitic pumice eruptions. The footwall of the Sumisu Rift has been uplifted 200-1700 m, and rift basement depth prior to 1.1 Ma exceeded 2 km. The basement of the rift is formed by early rift basaltic lavas and intrusives, as well as by arc pyroclastics metamorphosed to zeolite or lower greenschist facies. Intra-rift basaltic eruptions and rhyolitic eruptions were common but explosive arc volcanic activity dramatically increased 250 Ka. Unlike the forearc region, fluids other than sea water are not circulating locally through the sediments in the Sumisu Rift. TECTONIC SETTING AND EVOLUTION OF THE IZU-BONIN-MARIANA REGIONThe present-day tectonic configuration of the Izu-Bonin-Mariana region ( Fig. 1) comprises, from east to west: the trench; the forearc terrane, made up of an inner trench wall, an outer-arc high, a forearc basin, and a frontal-arc high; the active Izu-Bonin and Mariana island arcs; the Izu-Bonin backarc rifts and the actively spreading Mariana backarc basin and its remnant arc, the West Mariana Ridge; the Shikoku and Pa...