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 Pac ific Rise out to a seafloor age of 1 Ma has been carried out between 1 8-degrees and 19-degrees-S. It reveals that some left-stepping axial d iscontinuities have been migrating southward at rates an order of magn itude faster than the spreading rates (1000 mm/a or higher). These rap id migration events have left on the Nazca plate discordant features s triking nearly parallel to the ridge axis. A discontinuity with an off set of several kilometers has migrated in two stages at around 0.45 an d 0.3 Ma, and has left two large discordant zones consisting of a seri es of unfaulted, hummocky basins bounded to the east by short ridges o riented about N-S, oblique to the ambient 013-degrees fabric. The morp hology 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 cent ers. Between 18-degrees-35' and 19-degrees-03'S, the ridge axis is fla nked a few kilometers to the east by a prominent, sedimented ridge pre viously recognized as a recently abandoned ridge axis. The present rid ge segment steadily deepens and narrows southward, which suggests the abandoned ridge has been rafted onto the Nazca plate during the ultraf ast southward propagation of the ridge segment rather than by one disc rete ridge jump. By transferring Pacific lithosphere to the Nazca plat e, these migration events account for most of the asymmetric accretion observed (faster to the east). This process is consistent with the fe atures common to asymmetric spreading, namely the sudden onset or demi se of asymmetric spreading, and the ridge segment to ridge segment var iability. Because the discordant zones left by these rapid migration e vents are near-parallel to the ambient seafloor fabric, they are unlik ely to be detected by conventional bathymetry or magnetic surveys, and so-called ''ridge jumps'' may actually often represent ultrafast prop agation 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 Paci fic Rise relative to the Brunhes/Matuyama reversal (0.78 Ma) mostly re flects that ultrafast propagation of ridge segments has transferred a larger amount of the Pacific lithosphere to the Nazca plate at 18-degr ees-S than at 19-degrees-S. In keeping with the regional features of t he magnetic anomalies, we propose that an 8 to 10 km left-stepping dis continuity 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 rep eated small lateral steps of the ridge segments inside of the disconti nuities during ultra-fast propagation episodes, and may be the consequ ence of a significant replenishment of the magma reservoir between 17- degrees and 17-degrees-30'S during the past 1 m.y.