Frictional characteristics of oceanic transform faults: progressive deformation and alteration controls seismic style

Oceanic transform faults are inferred to be weak relative to surrounding oceanic crust and primarily slip aseismically. Neither their weakness nor tendency to creep are well-explained. We test the effects of fault-rock evolution on oceanic transform fault frictional strength and stability using direct-shear experiments (at room temperature, 10 MPa normal stress, and fluid-saturated conditions) on dolerite from the East Pacific Rise and natural fault rocks from the exhumed Southern Troodos Transform, Cyprus. Dolerites and cemented breccias are frictionally strong (μ = 0.52–0.85) and velocity-weakening (strength decreases with increasing slip velocity, characteristic of earthquakes). In contrast, matrix-rich chlorite-bearing fault breccias and gouges are frictionally weak (μ = 0.25–0.48) and velocity-strengthening (characteristic of stable creep). This transition implies that seismic behavior is controlled by degree of damage and alteration, such that earthquakes can nucleate within relatively intact oceanic crust, whereas fault segments of increased damage and chlorite content tend to slip aseismically