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ABSTRACT The North American Cordillera is an archetypal terrane collage formed by collisional tectonics, yet a margin-wide plate-tectonic reconstruction that unifies surface geology with the subducted oceanic lithosphere observed in Earth’s mantle by tomographic imaging does not exist. Surface geology–based regional tectonic models are not easily reconciled with each other or existing mantle-based reconstructions, which demand broad reinterpretation of robust geologic observations. Here, we applied a novel workflow by structurally unfolding subducted Farallon lithosphere using mantle tomography to constrain the tectonic history of the North American Cordillera. We reconstructed long-lived, east-dipping subduction below North America and an east-dipping intra-oceanic subduction zone above the offshore Insular superterrane prior to Late Cretaceous time (100–80 Ma). We propose recognition of a “Bridge River” oceanic plate that subducted eastward below western North America prior to Late Cretaceous time. We implemented our slab unfolding–derived plate reconstructions into tomographically filtered mantle convection forward models and found that Late Cretaceous Insular superterrane–North America collision reproduces observed mantle structure, whereas models involving Jurassic Insular superterrane collision do not. We compared our reconstructions against independent upper-plate constraints such as a new restoration of Cordilleran strike-slip faults, surface geology, and Insular superterrane paleomagnetic data and found reasonable correspondence. Insular superterrane collision and northward translation in our reconstructions coincide with transpressional shear zones in southern California through SE Alaska. Our results suggest that westward lateral slab movement after subduction below North America played a crucial role in mantle dynamics that potentially reconciles the geologic record with the deep mantle.
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Evaluating uncertainty in North American plate-tectonic reconstructions from lower-mantle slabs
ABSTRACT Tomographic images hold key constraints for North American Cordillera plate-tectonic histories, but models that prioritize surface geology (i.e., top-down) or mantle tomographic slabs (i.e., bottom-up) show disparate conclusions. Here, we highlight five key questions regarding assumptions and uncertainties for building North American Cordillera plate reconstructions from tomography: (1) Which seismic anomalies under North America should be interpreted as subducted slabs? (2) How should paleo–subduction zone absolute locations be determined from mantle constraints? (3) How should paleo–subduction zone polarities be determined from tomography? (4) How should slab constraints be integrated into absolute plate motion models? (5) How do inferences from lower-mantle slabs compare to surface geologic remnants of accreted terranes along western North America? The lower mantle under North America shows five main slab-like, faster-than-average seismic anomalies; of these, four anomalies are generally interpreted as subducted slabs, and the other anomaly is possibly a geophysical artifact. Comparison of interpreted subducted slabs from published tomography shows the interpreted slabs generally occupy similar mantle locations, but slab dips may be highly variable between tomographic data sets. Thus, caution is needed when inferring paleo–subduction zone polarity or absolute locations directly from slab dips. Absolute plate motion models of areas offshore western North America during the Jurassic–Cretaceous imply a relatively fast, northeast-moving Farallon oceanic plate that precludes a stationary, long-lived (>40 m.y.), west-dipping intra-oceanic subduction zone. However, long-lived east-dipping or bivergent (east- and west-dipping) intra-oceanic subduction zones are permissible. Alternative conclusions about North American Cordillera paleo–subduction zone polarities and terrane-slab correlations stem from slab reconstruction approaches based on “vertical slab sinking” and “nonvertical slab sinking” hypotheses; future tests of these hypotheses will improve our understanding of Earth’s mantle and its convective processes.
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- PAR ID:
- 10668893
- Publisher / Repository:
- Geological Society of America
- Date Published:
- Page Range / eLocation ID:
- 57 to 79
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Book Chapter:
- https://doi.org/10.1130/2025.2565(04)
