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doi:10.2204/iodp.pr.340T.2012

Introduction

During Integrated Ocean Drilling Program (IODP) Expedition 340T we conducted borehole logging at Atlantis Massif, an oceanic core complex (OCC) just west of the spreading axis of the Mid-Atlantic Ridge, 30°N. Seismic data for the 800–1400 m interval cored in Hole U1309D during IODP Expeditions 304 and 305 are the primary new information, whereas a number of ancillary measurements at the site also aim to address the question of where seawater penetrates and alters young oceanic lithosphere, thus hydrating a key chemical reservoir in the Earth. The detachment fault that controlled formation of Atlantis Massif (Cann et al., 1997; Blackman et al., 2002; Schroeder and John, 2004; Karson et al., 2006) is known to have localized fluid flow in a ~100 m thick zone of deformation/alteration that is exposed at the seafloor (Boschi et al., 2006; McCaig et al., 2010). Additional zones of (at least) past seawater circulation are indicated in deeper known or inferred fault zones or portions of olivine-rich troctolite intervals that are altered to serpentinite (Hirose and Hayman, 2008; Michibayashi et al., 2008; Beard et al., 2009). Seismic reflectivity observed in multichannel seismic (MCS) data throughout the Central Dome and Southern Ridge of the massif (Canales et al., 2004; Singh et al., 2004; Blackman et al., 2009) may correspond to interfaces between zones of past and/or present fluid flow and the surrounding rock. The main goal of Expedition 340T was to begin to test this hypothesis.

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