Outcome of Hard Rock Drilling in the Izu–Bonin Arc

― ― Abstract The Izu ­ Bonin arc has been the target of several hard rock drilling expeditions, including those associated with the Ocean Drilling Program （ ODP ） and the Integrated Ocean Drilling Program （ IODP ） , as well as a drilling survey for delineating Japanese continental shelves undertaken by Japan’s Ministry of Economy, Technology and Industry. In 1989, ODP Legs 125 and 126 successfully recovered cores from the Izu ­ Bonin forearc area and the backarc rift basin （ Sumisu Rift ） . These cores provided the first opportunity to investigate the early volcanic and tectonic history of the Izu ­ Bonin arc in the Eocene and Oligocene. They also provided the earliest volcanic products of the Sumisu Rift, which is highly vesicular basalt. Drilling for the Japanese continental shelf survey was conducted in the most reararc side of the Izu ­ Bonin arc. The cores from the reararc seamounts reveal that the across-arc variations in magma chemistry and age of volcanism observed along the reararc seamount chains continue further toward the spreading center of the Shikoku Basin. The Kinan Escarpment appears to be the westernmost （ i.e., most reararc ） location where a slab-derived geochemical signature can be recognized in the erupted magma. Three IODP drilling expeditions have been undertaken in the Izu ­ Bonin arc region. Exp. 350 drilled in a small basin between the reararc seamount chains. Slightly over 1.8 km of sediment of mostly volcanic origin was drilled. This core preserves a continuous magmatic record in the reararc area since cessation of spreading of the Shikoku Basin, and provides critical information about how the reararc volcanoes were reestablished after the middle Miocene. Exp. 351 and 352 aimed to study subduction initiation processes. Exp. 352 was conducted in the Izu ­ Bonin forearc. It recovered forearc basalt （ FAB ） and boninites associated with the seafloor spreading at subduction initiation. Based on their ages and geochemical characteristics, fast-and short-lived seafloor spreading is estimated to have occurred. Exp. 351 recovered ocean crust, which is interpreted to be the basement of the arc, from the Amami Sankaku Basin between the Kyushu ­ Palau Ridge （ ancient Izu ­ Bonin arc ） and the Daito Ridge （ Mesozoic remnant arc ） . This basement is similar in age and geochemistry to FAB, which implies the Izu ­ Bonin arc basement is ocean crust produced following the onset of subduction. This expedition also provided for the first time a continuous volcanic record