Insights into the internal structure and formation of striated fault surfaces of oceanic detachments from in situ observations (13°20’N and 13°30’N, Mid-Atlantic Ridge)

Friday, 19 December 2014: 4:00 PM
Javier Escartin1, Diane Bonnemains1, Catherine Mevel1, Mathilde Cannat1, Sven Petersen2, Nico Augustin2, Antoine Bezos3, Valérie Chavagnac4, Yujin Choi1, Marguerite Godard5, Kristian Haaga6, Cedric Hamelin6, Benoit Ildefonse5, John W Jamieson2, Barbara E John7, Thomas Leleu4, Christopher J MacLeod8, Miquel Massot-Campos9, Paraskevi Nomikou10, Jean-Arthur L Olive11, Marine Paquet1, Celine Rommevaux1, Marcel Rothenbeck2, Anja Steinführer2, Masako Tominaga12, Lars Triebe2, Muriel Andreani13, Rafael Garcia14 and Ricard Campos14, (1)Institut de Physique du Globe de Paris, Paris, France, (2)GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany, (3)University of Nantes, Nantes, France, (4)University of Toulouse, Toulouse, France, (5)University of Montpellier II, Montpellier Cedex 05, France, (6)University of Bergen, Bergen, Norway, (7)University of Wyoming, Laramie, WY, United States, (8)Cardiff University, Cardiff, CF24, United Kingdom, (9)Universitat de les Illes Balears, Palma de Majorca, Spain, (10)University of Athens, Faculty of Geology and Geoenvironment, Athens, Greece, (11)WHOI, Woods Hole, MA, United States, (12)Michigan State University, East Lansing, MI, United States, (13)University Claude Bernard Lyon 1, Villeurbanne, France, (14)University of Girona, Girona, Spain
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°20’N and 13°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°20’N OD is largely intact, with an undisrupted corrugated surface. An abrupt, continuous moat where the OD emerges from the seafloor sloping at ~12-18° continuously sheds rubble onto the OD fault plane, blanketing it. An apron surrounds the detachment dipping ~10-14° towards volcanic rift valley floor thus forming a thin wedge above the active OD fault, which uplifts hangingwall material. In contrast, the 13°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 ~2 km in the spreading direction. Flanks of individual lineations display fault planes extending ~20-100 m laterally with well-developed, extension-parallel striae. At 13°20’N. These fault surfaces are primarily basalt fault breccias and minor serpentinite. At 13°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.