Upper plate structure controls nearshore subduction mechanics in Cascadia.

Rheological and density contrasts in the Cascadia upper plate linked to accreted terranes systematically control slab depth, with deeper subduction beneath the coherent, higher-velocity Siletz terrane and shallower beneath lower-velocity faulted Siletz and Franciscan terranes.

• Why it matters: Along-strike upper-plate heterogeneity is common at subduction margins, but its direct role in shaping slab geometry and interseismic deformation is still uncertain.
• What they did: A high-resolution, shore-crossing 3D seismic tomography model from central to southern Oregon was used to map velocity contrasts tied to distinct accreted terranes, then numerical simulations tested viscoelastic end-member rheologies and density contrasts against the observed uplift and slab geometry.
• The result: The simulations show that rheological contrasts reproduce coastal uplift patterns, while density contrasts match the along-strike slab-depth variation, implying inherited upper-plate structure can be a first-order control on nearshore subduction mechanics.

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