Crustal structure of Atlantic fracture zones

Summary. A seismic refraction experiment was carried out in 1982 over, and in the vicinity of, the southern transform valley of the Charlie-Gibbs double Fracture Zone to measure the rate of crustal thinning normal to the southern transform valley axis. Three reversed lines were shot with OBS. Two were east-west; one parallel to the fault on normal crust and one along the fault. The third line was north-south normal to, and intersecting, the first two lines. Travel times were modelled by ray-tracing. Thin (4 km thick) anomalously low-velocity crust was detected in the fracture zone. Along the N-S line normal crust thinned over a distance of 40 km from 8 to 5 km towards the fracture zone where the belt of anomalous crust was 12 km wide. Several publications demonstrate that such thinning is not due primarily to the effect of cold lithosphere on accretion at the ridge-transform intersection; the thinning may be related to the supply and distribution of magma from an injection zone situated roughly at the centre of each spreading segment. A gravity profile along the N-S line was also analysed. The Bougue