Paper No. 6
Presentation Time: 3:30 PM


PORRITT, Robert W., University of Southern California, Department of Earth Sciences, 3651 Trousdale Parkway, Los Angeles, CA 90089,

The USArray component of Earthscope has illuminated the structure of the Cordillera on an unprecedented scale. The seismic observatory has facilitated implementation of novel methods to image both the velocity and discontinuity structure of the western U.S. In this contribution we present seismic methods implemented with USArray and discuss some of the more prominent observations including unusually thin lithosphere (~70km) throughout much of the Cordillera, several high wave-speed bodies in the asthenosphere, the Yellowstone plume imaged to sub-transition zone depths, and large-scale changes in the seismic anisotropy within the Basin and Range. Local, PI-driven arrays such as FAME, FACES, CAFÉ, COLZA and the Array of Arrays have produced detailed images of the currently subducting Juan de Fuca plate. SNEP and the Wallowa experiment have imaged regions where delamination is thought to be proceeding within the past ~10Ma. At the eastern edge of the Cordillera, the Bighorns Arch Seismic Experiment images crustal structure to better understand the uplift of the Rocky Mountains, and upwelling associated with the Rio Grande Rift is imaged with the La Ristra and SIEDCAR arrays. These observations are fueling a debate on the fate of the Farallon plate system, which has been subducting off the western U.S. coast for at least 150 million years. The three primary hypothesizes suggest that either the slab is sinking vertically, the plate fragmented during subduction, or peeling away of the flat slab has initiated multiple episodes of continental delamination. Each hypothesis has significant ramifications for the strength of the oceanic and continental lithosphere and on possible sources of the Yellowstone Plume. While the plume is clearly imaged to the base of recent tomographic models, high wave-speed anomalies in its ascent path indicate past interaction between the plume and the subducting plate. Possible modes of this interaction include either the plume-encouraging breakup of the slab and/or the slab forcing the plume to flow around it. The interaction of these major mantle features represents only the most recent of interpretations of USArray data and models. Multi-disciplinary efforts to understand these constraints are beginning to prove fruitful in the differentiation of these hypothesizes.