Side-scan sonar studies of North Atlantic fracture zones

Abstract

The Institute of Oceanographic Sciences has studied zones in 3 locations along the N mid-Atlantic Ridge and one on the Azores-Gibraltar plate boundary. 2 areas have short (~20 km) offsets and the others large ones (> 100 km). An important new observation is that probably all of these fracture zones are associated with en echelon , 20 km long scarps striking about 45 ° to the fracture zone. These scarps are believed to develop from oblique faults formed in the early stages of shearing. In addition, true transform faults associated with long, linear valleys occur in the large-offset fractures. Their appearance is similar to that of large continental strike-slip faults. The observed morphology and tectonics of the fracture-zone valleys generally accords with the description of Francheteau et al . (1976). The small-offset fracture zones have asymmetric cross sections which are attributed to the interplay of median valley and fracture-zone tectonics. Only one case of a possible intrusive ridge has been tentatively identified, in the inactive portion of Charlie-Gibbs Fracture Zone. An improved instantaneous rotation pole for Africa/Eurasia has been determined from the trace of the Gloria Fault on the Azores-Gibraltar plate boundary.