HR: 15:25h
AN: V33G-08    [Abstracts]
TI: Thin Spherical Shell Model of Global Asthenosphere Flow: Applications to Geochemical Segmentation of MORs and Seismic Anisotropy
AU: * Yamamoto, M
EM: michiko@geology.cornell.edu
AF: Cornell University, Earth and Atmospheric Sciences, Snee Hall, Ithaca, NY 14853 United States
AU: Morgan, J P
EM: jp369@cornell.edu
AF: Cornell University, Earth and Atmospheric Sciences, Snee Hall, Ithaca, NY 14853 United States
AU: Morgan, W J
EM: wjmorgan@princeton.edu
AF: Harvard University, Earth and Planetary Science 24 Oxford St. , Cambridge, MA 02138 United States
AB: Asthenosphere plume-to-ridge flow has often been proposed to explain both the existence of geochemical anomalies at the mid-ocean ridge segments nearest an off-axis hotspot, and the existence of apparent geochemical `provinces' within the global mid-ocean spreading system. We have constructed a thin-spherical-shell finite element model to explore the possible structure of global asthenosphere flow and to determine whether plume-fed asthenosphere flow is compatible with present-day geochemical and seismic observations. In this model, lubrication theory approximations are used to solve for the flow profile in the vertical direction, and a ~100-km-scale mesh is used to solve for the mean horizontal asthenosphere flow. At each mesh node, the asthenosphere thickness is set according to the age/thickness of overlying lithosphere. Asthenosphere is assumed to be brought up by mantle plumes, with `sinks' of asthenosphere at spreading centers where compositional lithosphere is made, at trenches (where some, but not much asthenosphere is entrained and dragged down by subducting lithosphere), and also a distributed sink of asthenosphere due to its cooling and attachment to the base of the aging and thickening oceanic lithosphere. Important model boundary conditions are plate velocities and the changing thickness of the asthenosphere/lithosphere at continental margins. We also assume that the strength of all the plume (hotspot) asthenosphere sources is equal to the sum of all the asthenosphere sinks, i.e. that the asthenosphere has a present-day steady-state thickness and hotspot fluxes have remained constant through time. In spite of these evident oversimplifications, the model appears to show considerable promise as a possible mechanism to explain observed patterns of MOR geochemical segmentation. Atlantic, Indian, and Pacific MOR isotope geochemistry can be fit well at both medium and long wavelengths by the predicted global asthenosphere flow pattern from distinct plume sources. Matching recent observations of seismic anisotropy is currently more problematic. Fossil spreading directions and present-day plate motions also appear to correlate with seismic observations - we are currently trying to isolate each of these effects in an inverse formulation. The model makes specific and sometimes surprising predictions about the global segmentation of ridge geochemistry.
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 3210 Modeling
DE: 3220 Nonlinear dynamics
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting