The East ridge system 28.5–32°S East Pacific rise: Implications for overlapping spreading center development

Abstract

We report here on geophysical data from the East ridge and surrounding areas of the large-offset overlapping spreading centers (OSCs) that accommodate Pacific-Nazca opening between 28.5° and 32°S. The East ridge overlaps and is offset from the West ridge system by ∼ 120 km, forming the largest known pair of OSCs. In this area spreading rates reach the fastest currently active on Earth of ∼ 149 mm/yr. Although the East ridge is composed of 4 morphologically defined segments separated by 3 small OSCs, other geophysical characteristics imply 1 upwelling segment. All the active ridge segments in this area (including the propagating tips of the East and West ridges) form relative topographic highs with respect to the flanking sea floor; however, identified abandoned ridge tips form deeps. We interpret these data in terms of a model in which the propagating segment represents an overshoot of a surficial rupture of the brittle lithospheric layer, only partially coupled to the diverging flow of a more broadly distributed ductile deformation zone (DDZ), surrounding the steady-state ridges and crossing the offset between the OSCs. The topographic high of the propagating segment may be maintained primarily by along-axis melt migration from the stable spreading segments rather than by direct upwelling from beneath the ridge. The large overlapping ridges are inherently unstable and continued extension causes the overlapping axes to become offset from the stably spreading segments, cut off from the supply of melt, and replaced by a new set. The failed rift tips, for a period of time, overlie the broad DDZ and preferentially undergo continued extension and subsidence. The DDZ surrounding the ridge axes may be very broad in this area because of the very fast spreading rate, creating a very thin lithosphere susceptible to perturbation by relatively small mantle heterogeneities advected near the ridge axis, leading to the formation of the smaller OSCs observed.