The research addresses the relationship between observations of mantle heterogeneity and the history of subduction for about the last 200 Myr. This work requires coordinated efforts in two fields, namely, theoretical geodynamics and global tectonics. Subducted slabs constitute the largest known density constrasts in the mantle. The largest-scale seismic velocity and geoid anomalies are not correlated with present-day plate tectonics, but are qualitatively similar to older (100-200 Myr B.P.) configurations of subduction. This suggests that the large-scale pattern of mantle convection is dominated by the long-term patterns of subduction. The project will test this hypothesis quantitatively for various convection models. Previous mapping of Pacific Basin convergency zones (0-180 Myr B.P.) will be expanded to include other important paleosubduction zones in the Indian Ocean Basin and elsewhere. Subducted plate ages, rates, and locations will be converted into a corresponding map for the injection of thermal buoyancy into the upper mantle. The trajectories of slabs due to mantle flow will be calculated using a well-developed analytical method for flow in spherical shells. Both chemically layered and viscosity layered convection models driven subducted buoyancy forces will be explored. These dynamical models for the fate of old subducted slabs will be rigorously correlated with observations bearing on density contrasts in the mantle, particularly the new observations of seismic heterogeneity, the gravity field, and plate velocities. By integrating knowledge of plate tectonic history with present- day geophysical observations and convection theory, this work will provide entirely new constraints on the large-scale nature of mantle convection.
