THE EAST RIDGE SYSTEM 28.5-32-DEGREES-S EAST PACIFIC RISE - IMPLICATIONS FOR OVERLAPPING SPREADING CENTER DEVELOPMENT

We report here on geophysical data from the East ridge and surrounding areas of the large-offset overlapping spreading centers (OSCs) that a ccommodate Pacific-Nazca opening between 28.5 degrees and 32 degrees S . The East ridge overlaps and is offset from the West ridge system by similar to 120 km, forming the largest known pair of OSCs. In this are a spreading rates reach the fastest currently active on Earth of simil ar to 149 mm/yr. Although the East ridge is composed of 4 morphologica lly defined segments separated by 3 small OSCs, other geophysical char acteristics imply 1 upwelling segment. All the active ridge segments i n this area (including the propagating tips of the East and West ridge s) form relative topographic highs with respect to the flanking sea fl oor; however, identified abandoned ridge tips form deeps. We interpret these data in terms of a model in which the propagating segment repre sents 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-sta te ridges and crossing the offset between the OSCs. The topographic hi gh of the propagating segment may be maintained primarily by along-axi s melt migration from the stable spreading segments rather than by dir ect upwelling from beneath the ridge. The large overlapping ridges are inherently unstable and continued extension causes the overlapping ax es to become offset from the stably spreading segments, cut off from t he supply of melt, and replaced by a new set. The failed rift tips, fo r a period of time, overlie the broad DDZ and preferentially undergo c ontinued 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 rela tively small mantle heterogeneities advected near the ridge axis, lead ing to the formation of the smaller OSCs observed. (C) 1997 Elsevier S cience B.V.