The Juan de Fuca anomaly (Figure A45) is located below western North America, is N-S trending and is interpreted to be the still-subducting slab at the Cascadia subduction zone. In tomographic studies it has been imaged to depths of 250-400km (Sigloch et al., 2008; Schmandt and Humphreys, 2011; Chu et al., 2012). The Juan de Fuca slab has been interpreted as the result of the reinstatement of normal subduction following the accretion of the Siletzia microcontinent, and a subsequent period of flat-slab subduction (Schmandt and Humphreys, 2011). Above the slab lies the Cascadian arc, where magmatism started at 45-40 Ma (Schmandt and Humphreys, 2011; Wells and McCaffrey, 2013). The slab has been interpreted to be broken up by the Yellowstone plume (Obrebski et al., 2010, Long 2016). Fragmentation of the slab presumably occurred just prior to the arrival of the plume at the surface, around 19–17 Ma (Obrebski et al., 2010. We adopt this as the age of the base of imaged slab. Some weak positive anomalies are imaged in the UU-P07 model towards the east and northeast, named the F1 and F2 slabs in Obreski et al. (2010) and Long (2016), which may represent the remnant slabs of 45-19 Ma subduction, separated from the Juan de Fuca slab by the Yellowstone plume.