Multiscale Roughness of Upper Mantle Discontinuities Inferred from the USArray: Dependence on Tomography Models

Abstract

We used 3D plane wave migration to image topography of the upper mantle discontinuities at 410 km and 660 km depth that defines the transition zone. We find both discontinuities have topography variation at all resolvable scales. In this paper we examine the dependency of discontinuity roughness on tomography models. We migrated a common USArray data set with a selection of regional, global, surface wave, and body wave tomography models to distinguish different scales of roughness. The objective is to appraise what features are potential artifact from using an inaccurate velocity model. We find that the largest-scale features depend on the choice of tomography model, while smaller-scale features appear to be almost completely independent of current generation models. We suggest that this observation is additional evidence of the existence of small-scale roughness on the upper mantle discontinuities not captured with the data sampling density of the USArray. We find all models based only on body wave travel times alone do not remove a continent scale offset of both discontinuities that correlates with the edge of the craton. We conclude that offset is an artifact linked to underestimation of wave speed in the upper mantle by pure body wave models. Models produced by joint inversion with surface wave dispersion data are less prone to this apparent artifact. The large-scale topography variation is consistent with rigid plate motion models of the subducted Farallon slab underneath North American. Smaller scale topography is found to have larger variation in regions where the vertical mantle flow through the transition zone is implied by transition zone thickness.