Dynamics of the North American Plate: Large-Scale Driving Mechanism From Far-Field Slabs and the Interpretation of Shallow Negative Seismic Anomalies

With a small fraction of marginal subduction zones, the driving mechanism for the North American plate motion is in debate. We construct global mantle flow models simultaneously constrained by geoid and plate motions to investigate the driving forces for the North American plate motion. By comparing the model with only near-field subducting slabs and that with global subducting slabs, we find that the contribution to the motion of the North American plate from the near-field Aleutian, central American, and Caribbean slabs is small. In contrast, other far-field slabs, primarily the major segments around western Pacific subduction margins, provide the dominant large-scale driving forces for the North American plate motion. The coupling between far-field slabs and the North American plate suggests a new form of active plate interactions within the global self-organizing plate tectonic system. We further evaluate the extremely slow seismic velocity anomalies associated with the shallow partial melt around the southwestern North America. Interpreting these negative seismic shear-velocity anomalies as purely thermal origin generates considerably excessive resistance to the North American plate motion. A significantly reduced velocity-to-density scaling for these negative seismic shear-velocity anomalies must be incorporated into the construction of the buoyancy field to predict the North American plate motion. We also examine the importance of lower mantle buoyancy including the ancient descending Kula-Farallon plates and the active upwelling below the Pacific margin of the North American plate. Lower mantle buoyancy primarily affects the amplitudes, as opposed to the patterns of both North American and global plate motions.