Shear zone development in serpentinised mantle: Implications for the strength of oceanic transform faults

Oceanic transform faults display fewer and smaller‐magnitude earthquakes than expected for their length. Several mechanisms have been inferred to explain this seismic slip deficit, including increased fault zone damage resulting in elevated fluid flow, and the alteration of olivine to serpentine. However, to date, these possible mechanisms are not supported by direct observation. We use micro‐ to kilometre scale observations from an exhumed oceanic transform fault in the Troodos Ophiolite, Cyprus, to determine mineral‐scale deformation mechanisms and infer likely controls on seismic behaviour of serpentinised lithospheric mantle in active oceanic transform faults. We document a range of deformation fabrics including massive, scaly and phyllonitic serpentinite, attesting to mixed brittle‐ductile deformation within serpentinite shear zones. The progressive development of a foliation, with cumulative strain, is an efficient weakening mechanism in scaly and phyllonitic serpentinite. Further weakening is promoted by a transition in the serpentine polytype from lizardite‐dominated massive and scaly serpentinites to chrysotile‐dominated phyllonitic serpentinite. The development of a foliation and polytype transition requires dissolution‐precipitation processes. Discrete faults and fractures locally crosscut, but are also deformed by, foliated serpentinites. These brittle structures can be explained by local and transient elevated strain rates, and play a crucial role in strain localisation by providing positive feedback for dissolution‐precipitation by increasing permeability. We propose that the evolution in structure and deformation style documented within the serpentinised lithospheric mantle of the Southern Troodos Transform Fault Zone is a viable explanation for the dominantly creeping behaviour and long‐term weakness of oceanic transform faults