High-resolution transition zone structures of the Gorda Slab beneath the western United States: Implication for deep water subduction

The current data set from US Array provides an unprecedented opportunity to investigate mantle transition zone structures beneath the western United States. We have made transition zone images with the Common Converted Point (CCP) stacking method. More than 9600 high quality receiver functions were stacked with reference to two different three-dimensional tomography models and a one-dimensional velocity model. Where the Gorda plate passes through the transition zone, the 410 discontinuity has been elevated similar to 25 km and the 660 discontinuity has been depressed similar to 35 km. We interpret the transition zone topography in terms of mineral physics results in several different ways, noting in particular that recent measurements on the Clapeyron slope for the ringwoodite-to-perovskite phase transition under dry conditions give a phase boundary slope of similar to-1.3 to -0.4 MPa/K. The similar to 35 km deflection of the 660 discontinuity observed in the receiver functions seems to be the evidence that the subducted slab can carry abundant water from the surface to the transition zone, and in the transition zone the water in the slab may be fully saturated (e.g. the water content is similar to 2.0 wt%). Analyses of the velocity perturbations in the tomography models and the transition zone thickness indicate that the deep water is likely well confined within the subducted slab. We infer that the presence of water in the subducted Gorda slab might have contributed similar to 15 km and the thermal anomaly in the slab might have contributed similar to 20 km to the depression of the 660 discontinuity.