A Discussion of Uncertainty and Assumptions in North American Cordilleran Plate Tectonic Reconstructions using Lower Mantle Slabs

Jonny Wu, University of Arizona
Spencer Fuston, EOG Resources
Lorenzo Colli, Bluethink

Abstract

Tomographic images are vital 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 on assumptions and uncertainties for building North American Cordilleran plate reconstructions from lower mantle slabs: (1) Which tomographic anomalies under North America should be interpreted as subducted slabs? (2) How should paleo-subduction zone absolute locations be reconstructed from present-day slab locations? (3) Reconstructing paleo-subduction zone polarities from tomography; (4) Integrating mantle constraints within absolute plate motion models; and (5) Correlation of deep lower mantle slabs to geologic remnants of accreted terranes along western North America.

We compare the five main slab-like, fast tomographic anomalies under North America; four of these anomalies are generally interpreted as subducted slabs and the other anomaly is likely a geophysical artifact. The interpreted slabs generally occupy similar locations in the lower mantle but slab dips are highly variable in various published tomography models, ranging from east-dipping, sub-vertical, or west-dipping. Thus, caution is needed when inferring the polarity or absolute locations of ancient subduction zones directly from slab dips. We discuss how absolute plate motion models of offshore western North America during the Jurassic-Cretaceous precludes a stationary, west-dipping intra-oceanic subduction zone but east-dipping or bivergent east- and west-dipping intra-oceanic subduction zones are permissible. We contrast 'vertical slab sinking' and 'non-vertical slab sinking' slab reconstruction approaches and show their alternative conclusions on North America Cordilleran paleo-subduction zone polarities and terrane-slab correlations.

Session

Cordilleran tectonics