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Geological Society, London, Special Publications; 1992; v. 65; p. 199-210;
DOI: 10.1144/GSL.SP.1992.065.01.15
© 1992 Geological Society of London

Fractures and Stress

Fracture permeability and alteration in gabbro from the Atlantis II Fracture Zone

D. Goldberg1, C. Broglia1 & K. Becker2

1 Borehole Research Group, Lamont-Doherty Geological Observatory, Palisades, NY 10964, USA
2 Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL 33149, USA

Geophysical logs recorded over 500 m in Ocean Drilling Program Hole 735B have been analysed to identify the in situ properties and the fracture permeability of basement rocks in the Atlantis II Fracture Zone. Hydrous alteration minerals were most abundant in the upper section of the hole. Packer tests also indicate the highest permeabilities in this zone. This interval is characterized by the presence of several open fractures imaged by the acoustic televiewer and by temperature log indications of fluid flow through transmissive fractures. Negative temperature gradient and large semblance anomalies were found to correlate well with the largest of the observed fracture zones. Most of the 70 identified fractures strike subparallel to the north-south fracture zone and dip steeply WSW. These fractures probably formed as a result of normal faulting on the transform side of the uplifted ridge in the Atlantis II Fracture Zone, where 735B was sited, rather than as a direct result of horizontal plate tectonic motion.