Asymmetric sea-floor spreading and a nontransform axis offset: The East Pacific Rise 20°S survey area

Abstract

Data obtained from the East Pacific Rise near 20°S provide an opportunity to make a detailed study of an asymmetrically spreading rise crest. During the past 2.4 m.y., crustal accretion has occurred here at 70 mm/yr on the west fland and 92 mm/yr to the east. Ocean-floor depths are also asymmetrically situated across the rise axis with the east flank being 100 to 150 m deeper than the west. These two asymmetries can best be explained by presuming the existence of a heat source located below the west flank of the rise. The axial ridge of the East Pacific Rise is offset 15 km right laterally across a region near 20.7°S that is a 30- to 40-km wide zone of shearing and not a well-defined transform fault. This offset appears to be the result of a small, discrete jump of the spreading center that occurred about 2 m.y. ago. Because of the nontransform nature of the small axis offset, the orientation of its trace recorded by ocean-floor bathymetry and magnetics is not determined by the relative motion of the Nazca and Pacific plates but by the relative motion between each individual plate and the location of the small axial offset. The trace of the small offset at 20.7°S and of apparently similar features occurring near 49°N and 26°N on the Mid-Atlantic Ridge form broad V's about the spreading axis. The trends of the arms of the V's may record absolute motion of individual plates as they move away from the locus of the small, non-transform, axial offsets.