Fluid-assisted grain size reduction leads to strain localization in oceanic transform faults.

Oceanic Transform Faults are major plate boundaries representing the most seismogenic part of the mid ocean ridge system. Nonetheless, their structure and deformation mechanisms at depth are largely unknown due to rare exposures of deep sections. Here we study the mineral fabric of deformed mantle peridotites - ultramafic mylonites - collected from the transpressive Atobá ridge, along the northern fault of the St.

Earth's oldest mantle fabrics indicate Eoarchaean subduction.

The extension of subduction processes into the Eoarchaean era (4.0-3.6 Ga) is controversial.