Upper plate structure controls nearshore subduction mechanics in Cascadia

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 continental crustal segmentation on nearshore subduction mechanics at the Cascadia margin. Using a high-resolution, shore-crossing 3D seismic tomography model from central to southern Oregon, we image sharp along-strike contrasts in seismic velocity associated with distinct accreted terranes. We show that the subducting slab is systematically deeper beneath the coherent, higher-velocity Siletz terrane and shallower beneath lower-velocity faulted Siletz and Franciscan terranes. Numerical simulations constrained by the tomographic velocities demonstrate that rheological contrasts, represented by viscoelastic end-member models, generate the magnitude and pattern of coastal uplift, while upper-plate density contrasts reproduce the observed variation in slab depth. These findings demonstrate that inherited upper-plate structure, despite being ignored in geodetic locking models, can exert a first-order influence on nearshore subduction mechanics by controlling the partitioning of interseismic deformation and contributing to along-strike variations in slab geometry.