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doi:10.2204/iodp.proc.304305.201.2009

Introduction

Oceanic core complexes (OCCs) are domal bathymetric highs and have been recognized along ultraslow to intermediate spreading ridges (e.g., Cannat et al., 2006; Ohara et al., 2007; Okino et al., 2004; Smith et al., 2006; Tucholke et al., 1998). OCCs expose gabbroic rocks on the seafloor through detachment faulting and are often associated with serpentinized peridotite.

The Atlantis Massif OCC is located at 30°N latitude on the Mid-Atlantic Ridge (MAR). The Atlantis Massif formed within the last 1.5–2 m.y. (Blackman et al., 1998, 2002). During Integrated Ocean Drilling Program (IODP) Expedition 304/305, a combined total of 1415.5 m was drilled through the footwall of the central dome of the Atlantis Massif OCC at Site U1309 (see the “Expedition 304/305 summary” chapter; Ildefonse et al., 2007). The core is dominated by gabbroic lithologies and is moderately altered overall. The alteration of the core decreases and varies in style downhole. The alteration at times is linked to structural features such as faults and shear zones. High-strain crystal-plastic deformation is located in discrete shear zones that vary in thickness from millimeters to a few meters (see the “Expedition 304/305 summary” chapter).

This report presents the initial results of a pilot study utilizing the technique of electron backscatter diffraction (EBSD) to analyze the structural contrast between the center and the edge of a gabbroic shear zone. Microstructures are studied with the overall aim of determining the controlling deformation mechanisms and the conditions of deformation within the shear zone. This work builds upon previous studies on oxide-rich shear zones performed by Agar and Lloyd (1997). The results of this study can be used to interpret whether these small shear zones have been involved in the uplift of the OCC or only represent minor phases of deformation inside the gabbro body during uplift and whether they are associated with late-stage injection of melts into the gabbroic body.

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