IODP Expeditions 304 & 305 Characterize the Lithology, Structure, and Alteration of an Oceanic Core Complex

More than forty years after the Mohole Project (Bascom, 1961), the goal of drilling a complete section through in situ oceanic crust remains unachieved. Deep Sea Drilling Project - Ocean Drilling Program (DSDP-ODP) Hole 504B within the eastern Pacific (Alt et al., 1993) is the deepest hole ever drilled into ocean crust (2111 mbsf), but it failed to reach lower crustal plutonic rocks below the pillow basalts and sheeted dikes. IODP Expeditions 309 and 312 eventually recovered the long-sought transition from sheeted dikes into underlying gabbros by drilling into very fast-spreading Pacific crust (Wilson et al., 2006). The lithology and structure of oceanic crust produced at slow-spreading ridges are heterogeneous (e.g., Cannat et al., 1997) and offer unique drilling access to lower crust and upper mantle rocks. After ODP Hole 735B penetrated 1500 m of gabbro at the Southwest Indian Ridge (Dick et al., 2000), IODP Expeditions 304 and 305 recently recovered just over 1400 m of little-deformed, gabbroic lower crust from a tectonic window along the slow-spreading Mid-Atlantic Ridge.