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

Area of Science:

• Geology
• Tectonics
• Mineral Physics

Background:

• Oceanic transform faults are key seismogenic plate boundaries.
• Deep fault structures and deformation mechanisms remain poorly understood due to limited exposure.

Purpose of the Study:

• Investigate the mineral fabric and deformation mechanisms of mantle peridotites from the Atobá ridge.
• Understand the processes governing the weakening and maintenance of oceanic transform faults at depth.

Main Methods:

• Analysis of ultramafic mylonites (deformed mantle peridotites).
• Microstructural and mineral fabric studies under lower oceanic lithosphere conditions.

Main Results:

• Identified fluid-assisted dissolution-precipitation creep as the dominant deformation mechanism.
• Observed grain size reduction via pyroxene dissolution and precipitation, enhancing strain localization.
• Demonstrated this mechanism operates at lower stresses than dislocation creep.

Conclusions:

• Fluid-assisted creep significantly weakens the oceanic lithosphere.
• This mechanism is crucial for the initiation and persistence of oceanic transform faults.