The deepest scientific oceanic borehole to date, Ocean Drilling Program (ODP) Hole 504B, has penetrated 2111 m below the sea-floor (mbsf) and over 1800 m subbasement, bringing it near the bottom layer of the oceanic crust. The site is located in 5.9-m.y.-old crust 200 km south of the Costa Rica Rift in the eastern Pacific (Figure 1). To continue testing the accuracy of seismic and pertrologic models for the layered structure of oceanic crust, the ODP scientific party returned to Hole 504 in January 1993, commencing Leg 148 of the project to retrieve samples from the borehole beyond the depths reached by previous drilling legs.
Earlier drilling in Hole 504 confirmed models for the structure of the uppermost 2 km of oceanic crust, and the site became a reference section for upper crustal petrology, geochemistry, hydrothermal alteration, and magnetic and physical properties [Becker et al., 1989] Results obtained during Leg 140, along with seismic evidence, had suggested that the next drilling leg could penetrate into the top of the deepest layer of crust [Dick et al., 1992]. Leg 148 drilling results indicate that Hole 504B has now penetrated into the seismic transition to layer 3—the deepest layer of crust; however, actual penetration into the gabbros, coarse-grained basaltic rocks that crystallized from the slowly cooled magma, and determination of how the lithologic and seismic transitions are related were prevented when the drill bit hit a fault and was left at the bottom of the hole.
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