The primary activity proposed is to map the east flank of the East Pacific Rise (EPR) between 15-20 S, from near the rise axis out to crust of age 4 Myr. Gravity, magnetic, side-scan sonar and bathymetric data will be collected with continuous coverage for the latter two data sets. The original motivation for focusing on this area was the observation of long wavelength plate motion-parallel gravity lineations which span the southern Pacific ocean basin west of the EPR. An earlier NSF-funded project mapped the ultrafast spreading EPR axis and its west flank out to approximately 4-6 Myr, so this effort would complete the mapping of this very exciting area. In this region, there are four major anomalous features: gravity lineations, abundant seamounts, asymmetric subsidence, and asymmetric spreading. Their coexistence near this part of the EPR must be more than coincidence. However, with the existing, mainly one-sided data coverage, it is difficult to distinguish whether these factors are simply a function of the ultrafast spreading at the southern EPR or whether they are related to some unusual properties of the Pacific Plate and it asthenosphere. The primary questions to be addressed are 1) Are there gravity lineations on the Nazca Plate similar to those observed on the Pacific Plate in this area? 2) Does small-scale convection in the Pacific asthenosphere bring about the anomalousy low subsidence of young Pacific seafloor? 3) Are the seamounts and seamount chains symmetrically distributed about the axis? 4) Do Nazca Plate seamounts continue to form, remain active, and change in morphology as those on the Pacific Plate? 5) What is the connection between the seamounts and the gravity lineations? 6) Can repeated rapid migrations of ridge axis discontinuities account for most (or all) of the asymmetric spreading? 7) How is the long-wavelength segmentation of the SEPR expressed off-axis?

Whatever is found in the E-flank coverage will provide important answers to these questions. If, for example, neither gravity lineations nor major seamount volcanism occurs on the E-flank, then the causes of the Pacific plate gravity lineations and seamounts are linked to each other and to the anomalously low subsidence rate (probably due to a thermal anomaly beneath the Pacific Plate). If, on the other hand, both lineations and abundant seamounts exist on the Nazca Plate, then their origins are most likely tied to the very high spreading rates, perhaps due to the very thin lithosphere extending away from the axial region. In this case, the asymmetry in subsidence would not be related to the cause of the gravity lineations. If seamounts are commonplace on the E-flank in the absence of gravity lineations, then their respective origins are independent of each other, with the gravity lineations most likely tied to the low subsidence rate of the Pacific Plate (perhaps by ridge-perpendicular convection). Conversely, if gravity lineations are observed on the E-flank without a seamount province, then these gravity anomalies are tied to the ultrafast spreading, while the seamounts may share an origin with the low subsidence.

In addition, the proposed work will provide continuous off-axis coverage at the critical transition between a long, magmatically robust section of ridge-crest and the starved section leading to the 20 40'S OSC. Finally, the survey and analysis will provide the geological ground truth for much of the MELT experiment.