Unraveling the geometry of the Farallon plate: Synthesis of three-dimensional imaging results from USArray

Abstract We compare 12 recent three-dimensional (3D) seismic imaging results that made extensive use of data from the Earthscope Transportable Array (TA). Our goal is to sort out what can be said about the geometry of the Farallon plate. Our main approach is 3D visualization using a kinematic plate motion model as a framework. Comparison of results from all 12 image volumes indicates that the results are most consistent with a single, coherent Farallon slab overridden by North American. The Farallon 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. The gently dipping section varies along strike. Under Washington the deflection is minor but to the south the slab flattens to become nearly horizontal beneath southern Idaho. There is a strong agreement that the high velocity anomaly associated with the slab vanishes under eastern Oregon. 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 processes linked to the Yellowstone system. We find that all results are consistent with a 4D kinematic model of the Mendocino slab window under Nevada and Utah. In the eastern US the larger scale models all show a lower mantle anomaly related to the older history of Farallon subduction. The link between the lower mantle and new results in the U.S. Cordillera lies under the High Plains where the required USArray coverage is not yet complete. Image volumes in a unified format are supplied in an electronic supplement.