Résumé
Oceanic detachment faults (ODs) are known to play a significant role in oceanic crustal accretion along slow-spreading ridges, and many display a poorly understood corrugated fault surface. The ODEMAR cruise (Nov-Dec'14) studied the 13 degrees 20'N and 13 degrees 30'N ODs along the Mid-Atlantic Ridge via extensive microbathymetric surveys with AUV ABYSS (GEOMAR), combined with geological observations and sampling using ROV VICTOR (IFREMER). The 13 degrees 20'N OD is largely intact, with an undisrupted corrugated surface. An abrupt, continuous moat where the OD emerges from the seafloor sloping at approximately 12-18 degrees continuously sheds rubble onto the OD fault plane, blanketing it. An apron surrounds the detachment dipping approximately 10-14 degrees towards volcanic rift valley floor thus forming a thin wedge above the active OD fault, which uplifts hangingwall material. In contrast, the 13 degrees 30'N OD is cut by recent high-angle faults, and is likely inactive. The OD fault is well exposed along these recent high angle fault scarps, and along mass wasting scarps. The OD fault displays individual microbathymetric lineations throughout the >150 m of fault zone thickness, that are traced up to approximately 2 km in the spreading direction. Flanks of individual lineations display fault planes extending approximately 20-100 m laterally with well-developed, extension-parallel striae. At 13 degrees 20'N. These fault surfaces are primarily basalt fault breccias and minor serpentinite. At 13 degrees 30'N the scarps cutting the detachment system reveal highly heterogeneous deformation, with phacoidal blocks of undeformed peridotite, gabbro, and basalt enclosed in anastomosing shear zones. Basalt often shows greenschist grade alteration, and is only present in the upper 50 m of the OD fault zone. In detail, OD faults are characterized by anastomosing zones of localized, strongly anisotropic deformation at different scales (m to km), bounding bodies of largely undeformed rock (basalt, gabbro, peridotite) elongated in the extension direction. Hangingwall material (basalt and dolerite) gets reworked into the fault zone in the green schist facies and effectively accreted to the footwall, both as fault breccia and as large blocks within the fault. Such fine-scale, in situ observations provide unprecedented insights into the three-dimensional and heterogeneous strain localization at OD fault zones.