Spreading center jumps and sub-axial asthenosphere flow near the Galapagos hotspot

Abstract

We have identified a sequence of small rise-axis jumps on the Cocos—Nazca spreading center between 93° and 95.5°W. The locus of jumps has migrated 150 km west along the rise axis, away from the Galapagos Islands, during the last three million years, at an average rate of 50 mm/year. The linear increase in jump distance during this sequence of jumps has resulted in a change in regional azimuth of the rise axis from about 085° to 095°. We visualize this sequence of jumps as a new rift propagating through the Cocos plate, forming a new Cocos—Nazca spreading center. The region affected by these rise jumps appears to correlate with an area of exceptionally high-amplitude magnetic anomalies. The high-amplitude region seems to result from Fe-Ti-rich (FeTi) basalts of high remanent magnetization. We speculate that the development of the new accretion axis and concomitant rise jumps are related to the flow of FeTi basalt-producing asthenosphere away from the Galapagos hotspot. The snout of anomalous asthenosphere has remained nearly stationary, with respect to the Galapagos hotspot, during the last 3 m.y. A northwestward component of flow, reflecting the southward position of the plume center with respect to the spreading axis, might explain why the new spreading center is developing along a more northwesterly azimuth. The rise jumps have resulted in the sort of pattern of asymmetric accretion which is required to substantiate the hotspot hypothesis for the origin of the Cocos and Carnegie ridges. Several other puzzling platetectonic phenomena may be explained by the propagating rift model developed here.