Introduction

Earthquake focal mechanisms show that MORs are everywhere under tension (Huang et al., 1986). As a result, as soon as young lithosphere is created by magmatic extrusion, intrusion and accretion, the forces of plate separation cause it to fracture (some of these fractures allowing emplacement of new melt – Chapter 6). This fracturing results in a wide range of tectonic features, from tensional fissures less than 1 m wide, through normal faults with horizontal and vertical offsets of tens to hundreds of metres, to detachment faults that cut right through the lithosphere and have offsets of several kilometres. Many of the characteristics of these fractures depend on the spreading rate through its effect on the thermal structure and hence rheology of the lithosphere.

Fissures

The smallest scale of fracturing occurs right at the ridge axis and consists of tensional fissures. The distributions of fissures have been mapped, amongst others, by Crane (1987) using deep-towed side-scan sonar and Haymon et al. (1991) using a towed camera system. Many aspects of fissuring were reviewed by Wright (1998).