[1] Expeditions 304 and 305 of the Integrated Ocean Drilling Program cored and logged a 1.4 km section of the domal core of Atlantis Massif. Postdrilling research results summarized here constrain the structure and lithology of the Central Dome of this oceanic core complex. The dominantly gabbroic sequence recovered contrasts with predrilling predictions; application of the ground truth in subsequent geophysical processing has B071031 of 25 produced self-consistent models for the Central Dome. The presence of many thin interfingered petrologic units indicates that the intrusions forming the domal core were emplaced over a minimum of 100-220 kyr, and not as a single magma pulse. Isotopic and mineralogical alteration is intense in the upper 100 m but decreases in intensity with depth. Below 800 m, alteration is restricted to narrow zones surrounding faults, veins, igneous contacts, and to an interval of locally intense serpentinization in olivine-rich troctolite. Hydration of the lithosphere occurred over the complete range of temperature conditions from granulite to zeolite facies, but was predominantly in the amphibolite and greenschist range. Deformation of the sequence was remarkably localized, despite paleomagnetic indications that the dome has undergone at least 45°rotation, presumably during unroofing via detachment faulting. Both the deformation pattern and the lithology contrast with what is known from seafloor studies on the adjacent Southern Ridge of the massif. There, the detachment capping the domal core deformed a 100 m thick zone and serpentinized peridotite comprises ∼70% of recovered samples. We develop a working model of the evolution of Atlantis Massif over the past 2 Myr, outlining several stages that could explain the observed similarities and differences between the Central Dome and the Southern Ridge.

Analysis of an in situ fault zone within the Atlantis Massif oceanic core complex (Mid-Atlantic Ridge) provides clues to the relevant deformation mechanisms and their temporal evolution within oceanic crust. IODP EXP304/305 drilled a succession of gabbroic lithologies to a final depth of 1415 meters below the sea floor (mbsf), with very high recovery rates of up to 100% (generally ~80%). We identified an intra-crustal fault zone between 720 and 780 mbsf in a section of massive gabbro, olivine gabbro, oxide gabbro units, and minor diabase intrusions. Of particular interest is the section between 744 and 750 mbsf, which unfortunately was marked by low recovery rates (17%). Electrical borehole-wall images show a 1-m-thick zone of east-dipping fractures within this interval, which is otherwise dominated by N-S dipping structures. Despite the high fracture density in this section, the hole walls are smooth, with rare breakouts, suggesting that the low recovery rate was due to a change in lithology rather than well conditions. The recovered rocks include ultracataclasite and possibly incohesive fault gouge that formed in the upper amphibolite regime, with mostly amphibole infill. Logging data suggest that the gabbroic rocks in this interval are rich in hydrous phases, consistent with increased amounts of amphibole found in the core. Equilibration temperature conditions of about 640°C were obtained for plagioclase clasts and aluminous actinolite, assuming a pressure of 200MPa. The permeability of the fault zone is in the range of 10-19 to 10-17 m 2. Although the permeability appears to be high within the fault zone relative to other parts of the section, it is no higher than that in typical lower crustal material. As a consequence, because brittle failure occurred at high temperatures, the fault zone was subsequently completely sealed by hydrous minerals, thereby preventing further fluid circulation and preserving water in the crust.

Electrical borehole wall images represent grey-level-coded micro-resistivity measurements at the borehole wall. Different scientific methods have been implemented to transform image data into quantitative log curves. We introduce a pattern recognition technique applying texture analysis, which uses second-order statistics based on studying the occurrence of pixel pairs. We calculate so-called Haralick texture features such as contrast, energy, entropy and homogeneity. The supervised classification method is used for assigning characteristic texture features to different rock classes and assessing the discriminative power of these image features. We use classifiers obtained from training intervals to characterize the entire image data set recovered in ODP hole 1203A. This yields a synthetic lithology profile based on computed texture data. We show that Haralick features accurately classify 89.9% of the training intervals. We obtained misclassification for vesicular basaltic rocks. Hence, further image analysis tools are used to improve the classification reliability. We decompose the 2D image signal by the application of wavelet transformation in order to enhance image objects horizontally, diagonally and vertically. The resulting filtered images are used for further texture analysis. This combined classification based on Haralick features and wavelet transformation improved our classification up to a level of 98%. The application of wavelet transformation increases the consistency between standard logging profiles and texture-derived lithology. Texture analysis of borehole wall images offers the potential to facilitate objective analysis of multiple boreholes with the same lithology.

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