The sedimentary/volcaniclastic record of subduction initiation and magmatic arc development at IODP Site U1438

The Izu-Bonin-Mariana (IBM) system is the focus of several 2013-2014 International Ocean Discovery Program (IODP) expeditions (350, 351 and 352) which were designed to answer questions about the fundamental plate-tectonic processes of convergent-margin initiation and crustal development in intra-oceanic settings. The primary objectives of IODP Expedition 351 included discovering the nature and origin the pre-subduction basement (lithosphere) in the Amami Sankaku Basin, and the subsequent history of magmatic arc initiation and evolution as recorded in the overlying stratigraphic section. These objectives were achieved by drilling a single, deep hole where a 1.61 km volcanic/sedimentary section was cored (Expedition 351 Scientists, in prep.). The Recent to Eocene stratigraphy at Site U1438 was subdivided into five lithostratigraphic units: four sedimentary units (I, II, III, and IV) overlying volcanic rocks designated as igneous Unit 1 (1461.1-1611.1 mbsf). Sedimentary Unit I (0-160.3 mbsf) comprised mainly hemipelagic mud with minor ash beds, whereas Sedimentary Units II (160.3- 309.6 mbsf), III (309.6-1361.4 mbsf) and IV (1361.4-1461.1 mbsf) were dominated by coarser (sand-gravel) marine volcaniclastic sediments. There are progressive downhole lithification and alteration of the volcaniclastics from Unit II into Unit III and local alteration zones throughout the underlying section as documented in core and thin section descriptions and XRD data.