Reconciling North American Tectonics with the Deep Mantle through Structurally Unfolding Subducted Slabs and Mantle Convection Modeling



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The western margin of North America has contributed to many geological paradims such as exotic terrane accretion, continental arc magmatism, subduction accretionary complexes, and Cordilleran strike-slip faulting. Yet, despite decades of research, the tectonics of this accretionary margin remain uncertain. The aim of this dissertation is to use an approach of structurally unfolding subducted slabs, imaged by mantle tomography, back to Earth’s surface paired with mantle convection modeling to address several uncertain aspects of western North American tectonics. We structurally unfolded upper mantle slabs below the Pacific Northwest and Alaska to build a “tomographic” plate reconstruction of Paleocene to recent western North America. These unfolded slab lengths, tested against near-trench magmatism, require an additional “Resurrection” tectonic plate between the Farallon and Kula plates offshore Paleocene-Eocene western North America. We identify fragments of the wholly subducted Resurrection plate in the upper mantle below Yukon, Canada. These unfolded slab lengths also constrain a Late Cretaceous margin-parallel slab break off event that we interpret as the collision of the Wrangellia Composite terrane (WCT), along an east-dipping intra-oceanic subduction zone, and a North American active margin. We use mantle convection modeling to generate predicted mantle structures for our new unfolded-slab reconstruction and those involving Jurassic WCT collision. Comparison to observed mantle structure demonstrates that our unfolded slab-derived plate reconstruction is the only model that fully reproduces all observed lower mantle slabs. We also show strong correlation between our reconstruction and a new restoration of Cordilleran strike-slip faults, WCT paleomagnetic data, and upper plate shear zone deformation. Our results show that it is possible to reconcile North American geology with the mantle record of subduction if lateral slab advection is allowed. Finally, I review the uncertainties inherent to building plate tectonic reconstructions from mantle tomographic constraints and propose several practical considerations that frame future western North American plate reconstructions.



North America, Tectonics, Mantle Tomography, Cordillera, Resurrection Plate, Chugach Terrane, Wrangellia Composite Terrane, Insular Superterrane, Intermontane Superterrane, Paleomagnetics, Franciscan Complex, Alaska, Sanak-Baranof Belt, British Columbia, Yukon, Slab, Unfolding


Portions of this document appear in: Fuston, S., and Wu, J., 2021, Raising the Resurrection plate from an unfolded-slab plate tectonic reconstruction of northwestern North America since early Cenozoic time: GSA Bulletin, v. 133, p. 1128–1140, doi:10.1130/B35677.1.