Outcrops of deeply derived ultramafic rocks and gabbros are widespread
along slow spreading ridges where they are exposed in the footwall of
detachment faults. We report on the microstructural and petrological
characteristics of a large number of samples from ultramafic exposures
in the walls of the Mid-Atlantic Ridge (MAR) axial valley at three
distinct locations at lat. 13 degrees N and 14 degrees 45'N. One of
these locations corresponds to the footwall beneath a corrugated
paleo-fault surface. Bearing in mind that dredging and ROV sampling may
not preserve the most fragile lithologies (fault gouges), this study
allows us to document a sequence of deformation, and the magmatic and
hydrothermal history recorded in the footwall within a few hundred
meters of the axial detachment fault. At the three sampled locations, we
find that tremolitic amphiboles have localized deformation in the
ultramafic rocks prior to the onset of serpentinization. We interpret
these tremolites as hydrothermal alteration products after evolved
gabbroic rocks intruded into the peridotites. We also document two types
of brittle deformation in the ultramafic rocks, which we infer could
produce the sustained low magnitude seismicity recorded at ridge axis
detachment faults. The first type of brittle deformation affects fresh
peridotite and is associated with the injection of the evolved gabbroic
melts, and the second type affects serpentinized peridotites and is
associated with the injection of Si-rich hydrothermal fluids that
promote talc crystallization, leading to strain localization in thin
talc shear zones. We also observed chlorite + serpentine shear zones but
did not identify samples with serpentine-only shear zones. Although the
proportion of magmatic injections in the ultramafic rocks is variable,
these characteristics are found at each investigated location and are
therefore proposed as fundamental components of the deformation in the
footwall of the detachment faults associated with denudation of
mantle-derived rocks at the MAR.