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1 Department of Earth Sciences, Cardiff University, Cardiff CF10 3YE, UK
2 Laboratoire de Geosciences Marines (CNRS UMR 7097), Institut de Physique du Globe, 75252 Paris, France
3 Department of Geosciences, University of Houston, Houston, Texas 77204, USA
4 Department of Earth Sciences, Cardiff University, Cardiff CF10 3YE, UK
5 Department of Earth Sciences, The Open University, Milton Keynes MK7 6AA, UK, and Department of Geology, University College Dublin, Dublin 4, Ireland
6 Department of Earth Sciences, Cardiff University, Cardiff CF10 3YE, UK
7 School of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK
8 Department of Earth Sciences, Cardiff University, Cardiff CF10 3YE, UK
9 Halliburton Inc., Pitmedden Road, Dyce, Aberdeen AB21 0DP, Scotland, UK
10 Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA
From a detailed survey and sampling study of corrugated massifs north of the Fifteen-Twenty Fracture Zone on the Mid-Atlantic Ridge, we demonstrate that their surfaces are low-angle detachment fault planes, as proposed but not previously verified. Spreading-direction–parallel striations on the massifs occur at wavelengths from kilometers to centimeters. Oriented drill-core samples from the striated surfaces are dominated by fault rocks with low-angle shear planes and highly deformed greenschist facies assemblages that include talc, chlorite, tremolite, and serpentine. Deformation was very localized and occurred in the brittle regime; no evidence is seen for ductile deformation of the footwall. Synkinematic emplacement of diabase dikes into the fault zone from an immediately subjacent gabbro pluton implies that the detachment must have been active as a low-angle fault surface at very shallow levels directly beneath the ridge axis. Strain localization occurred in response to the weakening of a range of hydrous secondary minerals at a very early stage and was highly efficient.
Key Words: detachment fault • oceanic core complex • mid-ocean ridge • Mid-Atlantic Ridge
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