EAST PACIFIC RISE 18-DEGREES-19-DEGREES-S - ASYMMETRIC SPREADING AND RIDGE REORIENTATION BY ULTRAFAST MIGRATION OF AXIAL DISCONTINUITIES

A detailed bathymetric, side scan, and magnetic survey of the East Pacific Rise out to a seafloor age of 1 Ma has been carried out between 18-degrees and 19-degrees-S. It reveals that some left-stepping axial discontinuities have been migrating southward at rates an order of magnitude faster than the spreading rates (1000 mm/a or higher). These rapid migration events have left on the Nazca plate discordant features striking nearly parallel to the ridge axis. A discontinuity with an offset of several kilometers has migrated in two stages at around 0.45 and 0.3 Ma, and has left two large discordant zones consisting of a series of unfaulted, hummocky basins bounded to the east by short ridges oriented about N-S, oblique to the ambient 013-degrees fabric. The morphology and reflectivity characteristics of these discordant zones are akin to the overlap basins and abandoned ridge tips which make up the migration trails of large, slowly-migrating overlapping spreading centers. Between 18-degrees-35' and 19-degrees-03'S, the ridge axis is flanked a few kilometers to the east by a prominent, sedimented ridge previously recognized as a recently abandoned ridge axis. The present ridge segment steadily deepens and narrows southward, which suggests the abandoned ridge has been rafted onto the Nazca plate during the ultrafast southward propagation of the ridge segment rather than by one discrete ridge jump. By transferring Pacific lithosphere to the Nazca plate, these migration events account for most of the asymmetric accretion observed (faster to the east). This process is consistent with the features common to asymmetric spreading, namely the sudden onset or demise of asymmetric spreading, and the ridge segment to ridge segment variability. Because the discordant zones left by these rapid migration events are near-parallel to the ambient seafloor fabric, they are unlikely to be detected by conventional bathymetry or magnetic surveys, and so-called ''ridge jumps'' may actually often represent ultrafast propagation of a ridge segment. Variations in fault azimuth with age show there has not been any significant change in spreading direction over the past 0.8 m.y. Instead, the counterclockwise trend of the East Pacific Rise relative to the Brunhes/Matuyama reversal (0.78 Ma) mostly reflects that ultrafast propagation of ridge segments has transferred a larger amount of the Pacific lithosphere to the Nazca plate at 18-degrees-S than at 19-degrees-S. In keeping with the regional features of the magnetic anomalies, we propose that an 8 to 10 km left-stepping discontinuity which was located between 17-degrees and 17-degrees-30'S at 0.78 Ma has been recently redistributed into the present staircase of small left-stepping discontinuities between 16-degrees and 19-degrees-S. This smoothing of the ridge geometry probably occurred through repeated small lateral steps of the ridge segments inside of the discontinuities during ultra-fast propagation episodes, and may be the consequence of a significant replenishment of the magma reservoir between 17-degrees and 17-degrees-30'S during the past 1 m.y.