EVIDENCE FROM THE OMAN OPHIOLITE FOR SUDDEN STRESS CHANGES DURING MELT INJECTION AT OCEANIC SPREADING CENTERS

THE system of dikes in the uppermost mantle below oceanic spreading ce ntres bears witness to the processes involved in the supply of melt to the ridge. Such systems cannot be studied on currently active ridges, but can be seen in ophiolites; in particular, the Oman ophiolite allo ws us to examine segments of a fast-spreading ridge1. Here we present a statistical analysis of the orientation of intrusions in the uppermo st mantle section of the Oman ophiolite spreading centres, which point s to the existence of two systems: dikes whose azimuth is parallel to the sheeted dikes (which we thus consider as related to the extensiona l lithospheric stress field) and sills, roughly parallel to the Moho2. Branching contemporaneous dikes and sills are frequently observed. We suggest that these relations can be explained by repeated sudden chan ges of the dominant stress field from lithospheric to asthenospheric. Such changes could result from the episodic relaxation of lithospheric stress in response to tension fracturing of the lithospheric lid, rel ated to sudden melt surges. This episodic behaviour is recorded in the sheeted dike complex, with each dike (each about one metre wide) repr esenting one of these fractures.