CRETACEOUS COLLISION RECONCILES WESTERN NORTH AMERICAN TECTONICS WITH DEEP MANTLE SLABS

Western North America is an archetypal terrane collage formed by collisional tectonics, yet a margin-wide plate tectonic reconstruction that unifies surface geology with the mantle record of subduction does not exist. Regional tectonic models based on geologic constraints (Saleeby and Dunne, 2015; Stevens Goddard et al., 2018) are not easily reconciled with mantle-based reconstructions that demand broad reinterpretation of robust geologic observations and rely on vertical slab sinking (Sigloch and Mihalynuk, 2013). Here, we constrain western North American tectonics by structurally unfolding subducted Farallon slabs using mantle tomography. We reconstruct long-lived, east-dipping subduction below North America and an east-dipping intra-oceanic subduction zone above the offshore Wrangellia Composite terrane (WCT) prior to Late Cretaceous time. Our slab unfolding-derived Cretaceous WCT collision reproduces observed mantle structure in tomographically filtered mantle convection forward models, whereas models involving Jurassic WCT collision do not. We test our reconstruction against a new restoration of Cordilleran strike-slip faults, upper plate shear zone deformation, and WCT paleomagnetic data. Our reconstructed WCT collision and northward translation coincides with transpressional shear zones in southern California through SE Alaska. Contrary to the “vertical slab sinking” paradigm, our results argue lateral slab movement plays a crucial role in mantle dynamics and, when accounted for, reconciles the geologic record with the deep mantle.