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doi:10.2204/iodp.pr.305.2005
Integrated Ocean Drilling Program (IODP) Expedition 305 is the second of a two-expedition program to drill and core a section of denuded lower crust, Atlantis Massif, located on the western flank of the Mid-Atlantic Ridge (MAR) at 30°N. Atlantis Massif formed within the past 1.5–2 m.y. and is the inside corner bathymeric high at the intersection between the MAR and the Atlantis Fracture Zone (Fig. F1). The corrugated, striated central portion of this domal massif displays morphologic and geophysical characteristics inferred to be representative of an oceanic core complex (OCC) exposed via long-lived detachment faulting (Cann et al., 1997; Blackman et al., 1998, 2004; Collins et al., 2001). The "core" of the complex is presumably composed of crust and upper mantle rocks denuded by a detachment fault exposed over an 8–10 km wide, 15 km long area that forms the elongate doubly plunging domal seafloor morphology (Fig. F1). An adjacent basaltic block to the east is interpreted as the hanging wall to the detachment fault. Initiation of a deep hole in the footwall of Atlantis Massif was achieved during IODP Expedition 304 at Site U1309. After drilling a pilot hole to 102 meters below seafloor (mbsf) in Hole U1309B, Hole U1309D (Fig. F2) was started and the upper 20 m was cased. During Expedition 304, Hole U1309D was cored to a depth of 401.3 mbsf, with overall recovery of 64%, dominantly gabbroic rocks. During Expedition 305, Hole U1309D was deepened to 1415.5 mbsf, again with high recovery (average = 74.8%) of dominantly gabbroic rocks. Hole U1309D was logged twice (~830–400 mbsf and ~1400–200 mbsf). Attempts during Expedition 304 to drill and core through the hanging wall and a detachment fault hypothesized to be present beneath it were unsuccessful (Shipboard Scientific Party, 2005).