IODP Drilling at the Godzilla Megamullion: The Nature of the Backarc Basin Lower Crust and Upper Mantle

― ― Abstract A significant fraction of the ocean floor is created in back-arc basins, where water plays a major role in generating back-arc basin basalts, contrasting strikingly with magmatic processes at mid-oceanic ridges. Furthermore, much of our understanding of all of the oceanic crust comes from ophiolites, which are largely attributed to supra-subduction zone environments. Therefore, studying the back-arc basin lower crust and uppermost mantle is arguably important, contribut-ing to the overall geology of the oceanic crust. The Godzilla Megamullion, located in the extinct Parece Vela Basin in the Philippine Sea, is the largest known oceanic core complex. It is an elongated massif with a distinct corrugated surface consisting of several individual domal highs. It records the secular evolution of the mantle melting beneath a dying back-arc spreading ridge along the length of the megamullion surface. Furthermore, strong heterogeneity in the P-wave velocity structure is observed along the length of the megamullion, with a normal oceanic crust-like structure in the distal （ i.e., near breakaway ） to medial parts, and a shallow high-velocity body in the proximal （ i.e, near termination ） part. The Godzilla Megamullion should arguably be the best place in the world to study the architecture of the back-arc basin lower crust and uppermost mantle, and the actual crust/mantle boundary through the International Ocean Discovery Program （ IODP ） . By locating three 400- to 800 m-deep drill holes along its length, key data are obtained to better understand and constrain the composition of the back-arc basin oceanic crust and uppermost mantle, as well as the architecture of oceanic core complexes. The extinct backarc basin environment at the Godzilla Megamullion provides a further unique opportunity to explore life in an oceanic crust after extinction of its hydrothermal activity.