UNRAVELING THE GEOMETRY OF THE FARALLON PLATE: SYNTHESIS OF THREE-DIMENSIONAL IMAGING RESULTS FROM THE USArray

We review recent imaging results related to the three-dimensional (3D) geometry of the subducted Farallon plate under North America. We compared twelve 3D seismic image volumes obtained from recent publications using 3D visualization and a 4D-kinematic plate motion model that was used to provide a framework for comparisons. We find no compelling evidence to reject the simplest model of the large-scale subduction process in which a coherent Farallon slab was overridden by the North American plate. The Farallon slab can be tracked from the trench in the Pacific Northwest to its remnants in the lower mantle under eastern North America. From the trench the lithosphere has a low dip to the volcanic arc. Immediately east of the arc the slab steepens sharply before undergoing a decrease in dip above the 410 km discontinuity that varies along strike. Under Washington the dip change at the 410 is minor but to the south the slab flattens to become nearly horizontal beneath southern Idaho at a comparable depth. The tomography models are in strong agreement that the high velocity anomaly vanishes under eastern Oregon and parts of Idaho. Scattered wave imaging results, however, suggest the top of the anomaly is continuous. These can be reconciled if one assumes the wavespeed anomaly has been neutralized by some process linked to the Yellowstone system. The results are found to be consistent with a 4D kinematic model of the Mendocino slab window under Nevada and Utah. To the east the larger scale models all show a strong, continuous anomaly in the lower mantle that has long been recognized as linked to the older history of Farallon subduction. The link between this unambiguous anomaly and new results in the U.S. Cordillera lies under the high plains where the required USArray coverage is not yet complete.