Résumé
Detachment faults that exhume mantle-derived peridotites play a significant role in plate divergence at slow-spreading ridges. These faults are assumed to root into the brittle-ductile transition (BDT). Significant weakening mechanisms allow these detachments to develop. Serpentine and other hydrous alteration minerals have been shown to localize strain in the upper, hydrothermally altered part of the detachment system. At magmatically active detachment-controlled ridges, previous studies indicate that detachments probably root into magmatic injections emplaced near the BDT. At the eastern part of the Southwest Indian Ridge, microseismicity indicates an axial brittle lithosphere up to 25 km thick (Schlindwein & Schmid, 2016), while geological investigations show that spreading is locally nearly amagmatic (Sauter et al., 2013). Using 99 dredged samples of partially serpentinized peridotites, we identify an episode of deformation that we infer occurred at the deep root of detachment faults in this nearly amagmatic end-member setting. This deformation is heterogeneous at the sample scale. It combines ductile and brittle mechanisms, with the development of extensively recrystallized anastomozing microshear zones. Estimates of deformation temperature (850-950 degrees C) and deviatoric stresses (90-325 MPa) are derived, respectively, from pyroxene thermometry and recrystallized grain size geopiezometry. We show that strain localization is initially controlled by the contrasted behaviour of orthopyroxene (primarily brittle with microfractures, kinks and local dynamic recrystallization) and olivine (primarily ductile with dislocation creep, extensive dynamic recrystallization, kinks and localized microfractures). Sample scale thermo-mechanical models with bimineralic composition (orthopyroxene and olivine) are used to investigate the role of this rheological contrast. In models as in nature, ductile shear zones in olivine initiate preferentially near brittle orthopyroxene.