Paper No. 292-9
Presentation Time: 9:00 AM-6:30 PM
MANTLE GEODYNAMICS AND LITHOSPHERIC PROCESSES AT THE EASTER AND JUAN FERNANDEZ MICROPLATES: CONSTRAINTS FROM GRAVITY ANALYSIS AND NUMERICAL MODELING
This study investigates crustal accretion and mantle geodynamics at microplates using mantle Bouguer anomaly (MBA) gravity calculations and exploratory numerical models. The focus of this investigation is the Easter and Juan Fernandez microplates, which are located in the eastern Pacific Ocean along the Pacific, Nazca, and Antarctic plate boundaries. Both microplates formed during chron 3N (5 Ma – 3.9 Ma) and rotate clockwise, at 15°/myr (Easter) and 9°/myr (Juan Fernandez). The study area also encompasses the Easter/Salas y Gomez mantle plume located near the Easter microplate. For both microplates, average MBA is different between the west and east ridge; the west ridges have more negative MBA values. This is consistent with the east ridges rifting into cooler, older lithosphere. The thermal effect of the Easter/Salas y Gomez plume as constrained by comparing MBA along the two microplates’ east and west rifts is minimal; the cool, rifting lithosphere on the east ridge may mask the hotspot gravity signal. The MBA calculations illustrate the importance of taking plate boundary geometry into consideration when using observational data to constrain plume properties. It is not sufficient to characterize plume-ridge interactions in the Easter area using a simple system consisting of the fast-spreading East Pacific Rise and an off-axis Easter/Salas y Gomez plume, because the presence of a microplate significantly affects MBA patterns in the region. In the second part of this study, a series of finite element numerical models were run to investigate plume-microplate interaction and to explore the dynamics of the Easter microplate – Easter/Salas y Gomez mantle plume system. An off-axis plume was assigned one of three locations around the microplate as well as one of two values of excess temperature. In all of the models the plume is advected in the equivalent of a southward direction, along the clockwise rotating microplate. This southward plume flow is inconsistent with published isotopic studies indicating that Easter/Salas y Gomez has northward preferential flow. Additional numerical modeling work as well as the collection of new isotopic and seismic data throughout the study area could help to elucidate geodynamical, geochemical, and geophysical processes of plume-microplate interaction.