90-8 Evaluating Uncertainty In North American Plate Tectonic Reconstructions From Lower Mantle Slabs

Abstract:

Tomographic images hold key constraints for North American Cordilleran 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 Cordilleran 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?; and (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 between interpreted subducted slabs from published tomography shows the interpreted slabs generally occupy similar mantle locations but slab dips may be highly variable between tomography. Thus, caution is needed when inferring paleo-subduction zone polarity or absolute locations directly from slab dips. Absolute plate motion models of offshore western North America during the Jurassic-Cretaceous implies a relatively fast, northeast-moving Farallon oceanic plate that precludes a stationary, long-lived (>40 m.y.a.), 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 on North America Cordilleran paleo-subduction zone polarities and terrane-slab correlations stem from slab reconstruction approaches based on ‘vertical slab sinking’ and ‘non-vertical slab sinking’ hypotheses; future tests of these hypotheses will improve our understanding Earth’s mantle and its convective processes.

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