Upper Crustal Control on Nearshore Subduction Mechanics in Cascadia

Asif Ashraf , Emilie Hooft, Douglas Toomey

Along-strike upper plate heterogeneity is widely observed at subduction margins, but its direct influence on slab geometry and interseismic deformation remains debated. Here, we investigate the effect of upper-crustal geological segmentation on nearshore subduction mechanics at the Cascadia margin. Using a high-resolution, shore-crossing 3-D seismic tomography model from central to southern Oregon, we image sharp along-strike contrasts in upper-crustal velocity associated with distinct accreted terranes. We show that the subducting slab is systematically deeper beneath the coherent, high-velocity Siletz terrane and shallower beneath low-velocity faulted Siletz and Franciscan terranes. Numerical simulations constrained by the tomographic velocities demonstrate that upper-plate density and rigidity contrasts alone reproduce the observed variation in slab depth and, when combined with viscoelastic deformation, explain the magnitude and pattern of coastal uplift. These findings establish that the along-strike changes in upper-crustal backstop, despite being ignored in geodetic locking models, are a primary mechanical filter that organizes subduction zone architecture and regulates the accumulation of megathrust related strain.