Crustal structure of Atlantic fracture zones — I. The Charlie-Gibbs Fracture Zone

• Published in: Geophysical Journal of the Royal Astronomical Society Vol. 85; no. 1; pp. 107 - 138
• Main Authors: Whitmarsh, R. B., Calvert, A. J.
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Publishing Ltd 01.04.1986
• Subjects: Atlantic ocean; crustal structure; fracture zone
• ISSN: 0956-540X; 0016-8009; 1365-246X
• DOI: 10.1111/j.1365-246X.1986.tb05174.x

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 Bouguer anomaly was modelled, subject to the constraints of the seismic ray-tracing, after applying corrections for the three-dimensional bathymetry near the profile and for the lithospheric anomaly due to the age contrast across the northern fracture zone. The gravity models supported the seismic interpretation and also indicated low densities within the fracture zones. Two alternative models were obtained for the structure under Hecate Bank, an east-west ridge which lies within the crust generated at the short central spreading segment of the CGFZ. One model features dense material within the crust, the other normal but uplifted crust. The evidence favours the latter mode but the cause of the uplift is uncertain. The uplift may have accompanied a hypothetical spreading direction adjustment and/or jumps in the location of the short central spreading axis.