Constraining fault friction in oceanic lithosphere using the dip angles of newly-formed faults at outer rises

- A catalogue of dip angles for newly formed faults in oceanic lithosphere is presented.
- Nodal-plane dips are concentrated between 30 and 60°.
- There is a clear peak in the distribution at ∼45°.
- The formation of new faults at ∼45° is indicative of a low coefficient of friction.

We investigate the mechanical properties of the oceanic lithosphere using earthquake focal mechanisms from subduction zone outer rises. We study regions where faulting oblique to the pre-existing mid-ocean ridge fabric implies the formation of new faults. The nodal-plane dips of dip-slip earthquakes on these faults are dominantly in the range 30-60°, with a strong peak concentrated around 45°. This distribution is inconsistent with the pattern that would result from high coefficients of friction (e.g. 0.6, equivalent to normal faults forming at 60° and thrust faults at 30°). We instead suggest that the observed distribution of dips implies that faults in the oceanic lithosphere have a low coefficient of friction, due to either low-friction clay minerals formed by hydrothermal alteration at the ridge, or due to an intrinsic level of friction that is lower than that suggested by laboratory studies.