Seismic and Geodynamic Investigation of the Interaction Between the Oceanic Lithosphere and Upper Mantle

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This is a collaborative proposal between seismologists and geodynamicists to address three major questions. (1) What is the temperature structure of the Pacific lithosphere and upper mantle? (2) What are the dynamical causes of some ofthe more prominent temperature variations? In particular, are the dynamics controlled largely by near-surface physical processes (e.g., thermal boundary layer instabilities or TBI), by processes whose genesis lies deeper within the earth (hot-spot plumes, large-scale upwellings), or by interactions between deep-seated and shallow processes (e.g., plumes exciting thermal boundary instabilities)? (3) What complementaryinformation is available in radial and azimuthal anisotropy? These questions are motivated, in part, by a preliminary model of the thermal structure of the Pacific that reveals that the Pacific lithosphere has experienced a punctuated cooling history, cooling diffusively for its first 70 Ma and then reheating in the Central Pacific between ages of 70 and 100 Ma predominantly at depths between 70 and 150 km. New geodynamical simulations show that TBI forms naturally as the plate cools and may play an important role in the observed cooling history of the Pacific plate.The seismological research will continue to develop data sets and theoretical methods to improve the seismic and temperaturemodels of the oceanic upper mantle. The geodynamic research will aim to understanding the dynamic interplay between the oceanic lithosphere and upper mantle. Particular emphasis will be placed on how the two-stage cooling inferred for the Pacific lithosphere may occur, what controls it, and why it may not occur beneath other oceans. Particular interest is in how TBI initiate and evolve, how plumes erode and reheat the lithosphere and on what controls the efficiency of this process, and the interrelation between TBI and thermal plumes.A graduate student and undergraduate researchers will work at the interface between geodynamical and seismological modeling. In addition, the research will provide a synoptic view of the oceanic upper mantle that will be valuable for a number of new and on-going initiatives such as Margins, Ridge2000, and Ocean Mantle Dynamics (OMD).