Magnetic structure of the Juan de Fuca-Gorda Ridge Area

Abstract

By using the fracture pattern deduced by N. Pavoni (1966) with a different interpretation of the magnetic anomaly lineations, a reconstruction can be made in which the Juan de Fuca and Gorda ridges form a single, continuous, north-south trending feature. From this reconstruction, the present magnetic structural pattern can be derived by (1) development of northwest-southeast oriented zones of weakness. (2) left-lateral motion along a system of northwest-southeast faults in the area of the Blanco fracture zone, (3) clockwise rotation and left-lateral motion of blocks north of the Blanco fracture zone, and (4) continued left-lateral offset along the northwest-southeast faults, in the course of which 250 km of the original ridge crest was destroyed and the individual faults were compressed into the present Blanco fracture zone. This interpretation explains the present magnetic structural configuration and elucidates some of the problems involved in the application of the transform-fault concept to this area. The anomalous structural fabric (which may be related to the coastal geology) and the agreement of the postulated motion of the Gorda block along the Mendocino fracture zone with known motion on the San Andreas fault zone suggest that the Blanco fracture zone may have formed and the Juan de Fuca and Gorda ridges may have separated during the Late Tertiary coastal orogeny.