TOMOGRAPHIC EVIDENCE FOR CONTINENTAL-SCALE SUTURING IN NORTH AMERICA – A METHODOLOGY FOR OROGENS GLOBALLY

Highly resolving seismic tomography of the mantle beneath North America reveals evolving configurations of arc-trench positions back to the breakup of Pangea. Images of subducted oceanic lithosphere in the mantle can be combined with quantitative plate reconstructions to show where and when westward drifting North America overrode pre-existing, intra-oceanic subduction zones, accreting their associated arcs and basement terranes in Jurassic and Cretaceous times. Tectonic predictions from this “tomographic time machine” can be checked against the geological accretionary record of the Cordilleran orogen. Our study offers a blueprint for similar, continental-scale investigations in other accretionary orogens.

To demonstrate, we propose a resolution to the longstanding debate of how and when western North America accreted the microcontinental Insular Superterrane (Wrangellia, Alexander, Peninsular) and its southern relative, the Guerrero Superterrane. Mantle structure supports an unconventional paleogeography whereby the Mesozoic arcs had grown in a long-lived archipelago located 2000-4000 km west of Pangean North America, its paleo-trench lines marked by massive, steep, lower-mantle slab walls >10,000 km long. North America converged on the two microcontinents by westward subduction of the intervening Mezcalera and Angayucham Oceans, culminating in diachronous suturing between ~150 Ma and ~50 Ma. Hence geophysical subsurface evidence negates the widely accepted “Andean-style” model of Farallon-beneath-continent subduction since at least 180 Ma.

The geological record from Alaska to Mexico also supports archipelago paleogeography. Suturing of the Mezcalera and Angayucham Oceans is evident in a trail of collapsed, Jura-Cretaceous basin relics that run the length of the Cordillera.