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

Abstract

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 ɛ 1 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 ɛ 1 is calculated with respect to rifting obliquity for different amounts of stretching using continuum mechanics. For low stretching factors, typical of incremental seismic deformations, ɛ 1 obliquity is two times lower than rifting obliquity. For higher stretching factors, the stretching and spreading directions become parallel.