IODP site survey for drilling Godzilla Mullion: preliminary report of R/V Hakuho KH07-2 Leg 2 & 4 cruise

Miocene seafloor in the Parece Vela Basin (PVB) in the Philippine Sea is characterized by rugged topography and mantle exposure, indicating a weak magma supply in an extinct backarc basin. The PVB is unique in that the world's largest oceanic core complex (OCC), Godzilla Mullion, occurs here along a fast/intermediate-spreading ridge. A proposal to drill Godzilla Mullion is currently in the IODP review system. A total of 13 expeditions were conducted in the PVB so far. These expeditions included the recent dredging cruise KR03-01, submersible dive cruise YK04-05-Leg 4, and the seismic study cruises by the Japanese government for the Law of the Sea project. An IODP site survey was conducted during August to September 2007 as a two-leg cruise (KH07-2-Leg 2 and 4) of R/V Hakuho in the PVB. The primary purpose of the cruise was to characterize Godzilla Mullion in more detail and to locate better drill sites on it. The primary survey item was multiple dredge hauls (12 hauls have been completed by the end of Leg 2) on Godzilla Mullion so as to map lithological variations of the OCC footwall in more detail. One dredge haul (D5) recovered fresh glassy basalts without gabbros and peridotites from a possible breakaway of an individual dome-like high on Godzilla Mullion, suggesting a possibility that Godzilla Mullion may have evolved through multiple detachment events, not a single large detachment. A deep-towed proton magnetometer survey along the flow line of Godzilla Mullion was also conducted to better characterize the exhumation history of the OCC.