Oblique rifting at oceanic ridges: Relationship between spreading and stretching directions from earthquake focal mechanisms

The relationship between spreading and stretching directions is investigated at oblique-spreading oceanic ridges using earthquake focal mechanisms. The stretching direction at ridge axes corresponds to the direction of the greatest principal strain ɛ1 taken as the mean trend of the seismic T-axes of extensional earthquake focal mechanisms. It is compared with the spreading direction provided by global plate-motion models. We find that the stretching direction trends approximately halfway between the spreading direction and the normal to the ridge trend, a result in line with analogue experiments of oblique rifting. This result is satisfactorily accounted for with an analytical model of oblique rifting, for which the direction of ɛ1 is calculated with respect to rifting obliquity for different amounts of stretching using continuum mechanics. For low stretching factors, typical of incremental seismic deformations, ɛ1 obliquity is two times lower than rifting obliquity. For higher stretching factors, the stretching and spreading directions become parallel.