Site U1437: Expedition 350

Abstract

International Discovery Program (IODP) Site U1437 is in the Izu-Bonin-Mariana (IBM) arc, which formed in response to subduction of the Pacific plate over the past 52 My. The IBM arc is divided into two major segments, the Mariana arc and the Izu-Bonin arc. The Izu-Bonin arc is further divided into the Izu and Bonin segments (in the north and south, respectively), separated by a fault zone referred to as the Sofugan Tectonic Line; because these segments have somewhat different histories, we refer to them separately as the Izu arc and the Bonin arc. Site U1437 is located in the Izu rear arc, behind the main chain of active volcanoes (referred to as the arc front), about 90 km west of the arc-front Myojinsho Volcano and 330 km west of the axis of the Izu-Bonin Trench at 2117 meters below sea level (mbsl). Site U1437 is about 160 km west-southwest of IODP Site U1436, which was drilled as a 150 m geotechnical hole in preparation for Site IBM-4. Site U1437 is the first site drilled in the rear part of the Izu arc; all other sites have been in or near the Izu arc front or in the Izu fore arc, leading to an incomplete view of Izu arc magmatism as a whole. Site U1437 was selected as the ideal spot for drilling in the rear arc for two main reasons. First, the site is located in a basin between two rear-arc seamount chains (Manji and Enpo seamount chains in a spot that appears to be bathymetrically shielded from arc-front volcaniclastic density current inputs. This site was thus chosen to maximize the chances of the stratigraphic record being dominated by rear-arc volcaniclastic input. Second, the Neogene Izu rear-arc seamount chains are inferred to young eastward and onlap eastward onto Paleogene (Eocene to Oligocene?) arc rocks, referred to as basement rocks. At Site U1437, the Paleogene crystalline basement rocks are inferred to lie about 2200 meters below seafloor (mbsf) (as described below); this depth is probably attainable by the R/V JOIDES Resolution, although it was the deepest target ever proposed for the ship.