Imaging the complex Farallon subduction system with USArray derived joint inversion of body waves and surface waves

The 150 million year history of subduction of the Farallon plate is being well elucidated by the passage of USArray. In this study, we use body wave relative delay times to generate independent P, SV, and SH relative velocity models for the USArray footprint. In addition, we use Rayleigh wave phase velocities derived from teleseismic earthquakes and ambient seismic noise to constrain the lithospheric structure where body waves have limited crossing ray information to form the SV-joint velocity model. The model volume contains a complex series of high velocities mostly along a planar front representing the remnants of the Farallon plate system. This feature has significant lateral and radial extent; beginning off the western coast of the US and terminating east of the model resolution, which goes to the Mississippi river. The bottom of the slab is well imaged through the mantle transition zone to at least 1000km. However, low velocity anomalies within this plane show the slab is far from a continuous sheet. Low velocities break up the slab into several major provinces, relating to different ages of orogens and an episode of flat slab subduction. Additionally, high velocities are often imaged well above the trace of the top of the slab with similar anomaly amplitude and dip as the main slab. While many of these anomalies have been interpreted as mantle drips, the similarity to the slab suggests a possible subduction origin for the features. However, the relatively shallow depths of these features require some mechanism of differentiation to develop neutral buoyancy. The prevalence of these high velocities, such as the Siletzia Curtain, Isabella Anomaly, Nevada Anomaly, and a newly imaged feature under southwest Texas, suggests a differentiation mechanism is fairly common among plates subducting under North America allowing for the observation of widespread shallow high velocity anomalies.