Gorda plate motions from magnetic anomaly analysis

Abstract

Significant variations of spreading rates and directions in space and time along the Gorda ridge can be determined by analysis of magnetic anomalies. Converted to motion vectors, they suggest the existence of at least two and possibly three Gorda plate segments. Assuming rigid plate tectonics, present motion between the northern and two southern segments should be NW-SE right-lateral at around 3 cm/yr and any fault between them should intersect the ridge near 42°N. A linear bathymetric trough may represent at least part of such a proposed fault zone, named here the Gorda fault zone. At the continental margin, the northern Gorda plate segment has probably continued to converge obliquely for at least 5 m.y. However, the southern Gorda plate segments may have recently ceased to subduct and now be moving almost parallel to the slope. This seems to be in accord with seismological and geological evidence near and north of Cape Mendocino. In the future, rapidly decreasing spreading on the southern Gorda ridge may effectively lock the southern Gorda plate segment to the Pacific plate so that the present Cape Mendocino triple junction will transfer northwards to a point on the margin near 41°30′N.