Recent dueling propagation history at the fastest spreading center, the East Pacific Rise, 26°–32°S

Abstract

The East Pacific Rise (EPR) 26°–32°S, located between the Easter microplate and the Juan Fernandez microplate, has the world's fastest spreading rate and tectonics characterized by dueling ridge propagation. A GLORI‐B and Sea Beam 2000 side scan/bathymetry survey, together with other geophysical data collection, was conducted aboard the R/V Melville covering an approximately 5° by 5° area centered on this part of EPR. The side scan and bathymetric data reveal the complete geometry of the propagator system. The western and eastern ridges both curve inwardly and are overlapped ∼120 km with an offset to overlap ratio of nearly one. Several abandoned ridges and rifts are clearly mapped on the east flank, and a few failed rifts are also mapped near the western ridge tip. A fracture zone discovered south of Easter Island indicates a stage of stable ridge‐transform intersection in the past. An inversion with magnetic anomaly vectors was carried out for the area. A resultant magnetic boundary strike map was interpreted with the downward component profiles to construct a magnetic isochron map. The spreading and propagation rates were estimated from the magnetic isochrons. The net southward propagation rate of the western ridge is ∼120 km/m.y. for the last 1.9 m.y. The spreading rate of the eastern ridge has been ∼150 km/m.y., and its current propagation rate is estimated as ∼500 km/m.y. toward the north. Intense asymmetric spreading as high as 30% is observed at both ridges. Together with the side scan and bathymetric data, the magnetic lineations provide significant constraints on the evolution of the propagator system, and a propagation model with cyclic rift failure was applied to model the tectonic evolution during the last 2 m.y. A normal ridge‐transform intersection evolved into a nontransform overlapping offset ∼1.95 Ma, and the eastern and the western ridges have propagated alternately. The dueling propagation history is characterized by the dominant southward propagation of the western ridge and the transitory propagation events of the eastern ridge with a variety of overlap lengths, widths, and ridge curvatures in the transition zone.