Journey of the Insular micro-continent through accretionary, collisional and translational regimes in the North American Cordillera since 170 Ma: a tomotectonic case study.

Tomotectonics hindcasts paleo-trenches, through the spatiotemporal superposition of subducted lithosphere (slabs imaged in the earth’s mantle) with plate reconstructions (constrained by seafloor isochrons). The two geophysical datasets are linked through the tomotectonic null hypothesis, that oceanic lithosphere sinks vertically down after entering in the mantle. This linkage permits simple and testable predictions about the location and lifespan of volcanic arcs, and specifically about arc-continent collisions, switches in subduction polarity, and switches from consuming to transform plate boundaries. In a second stage, tomotectonics uses land geological observations from the accretionary orogen in order to test predictions arising from the geophysical data sets.

We have applied the tomotectonic method to the North American Cordillera, where lower-mantle slab geometries indicate the nearly simultaneous initiation (~200-180 Ma) of three intra-oceanic archipelagos in the northeastern proto-Pacific (figure: MEZ, ANG, and CR slabs). Westward subduction beneath 10,000 km-long MEZ and ANG pulled North America from Pangaea, opening the Central Atlantic. Coeval eastward convergence of Farallon plate beneath intra-oceanic CR is predicted from Pacific seafloor isochrons. This configuration of subduction zones facing each other across an archipelago is analogous to today’s southwest Pacific, where Australia, embedded in Indian/Tethys Ocean floor, and the Pacific Ocean are drawn in by double-sided subduction.

Each slab must be associated with a paleo-arc. Central and controversial in formation accounts of the Cordilleran has been the Insular microcontinent (INS, comprising Peninsular, Alexander, Wrangellia superterranes of Alaska and B.C.) and its southward extension of Guerrero superterrane (GUE) of Mexico. When, where and in what style did MEZ accrete to North America? Did INS subsequently translate thousands of kilometres along the margin (the “Baja-BC” debate between geology and paleomagnetism)? How did INS unite with the remainder of accretionary terranes that form Alaska?

We demonstrate how tomotectonics hindcasts the INS journey. Massive MEZ slab wall fixes INS-GUE’s initial, stationary, offshore position – in an accretionary regime. Full consumption of North American oceanic lithosphere, pulled beneath INS-GUE arcs, caused diachronous collision from ~155 Ma to ~90 Ma (Nevadan-Sevier deformation), leaving a trail of collapsed basins. Subduction was gradually forced outboard of MEZ: flip to Farallon subduction, eastward beneath INS-GUE (now attached to North America), brought another accretionary episode of Franciscan and Chugach subduction complexes, linked to Sierra Nevada and Coast Mountain batholith arcs.

Northward translation of INS by ~2000 km between 90-50 Ma (the “BajaBC” regime) corresponds with a lack of subduction (slab) beneath the paleo-margin. A key result is that both tomotectonics and paleomagnetic observations, which are completely independent, support large-scale translation.

Simultaneously, INS and North Americal collided obliquely with Central Alaska and Farallon arcs in a second collisional phase ~100-50 Ma, again in double-sided subduction. Since 170 Ma, Insular micro-continent experienced all regimes of modern double-sided archipelagos: subduction accretion, collision, subduction flip, and transform. 

 

Reference: Sigloch, K. & Mihalynuk, M.G. (2025), Tomotectonics of Cordilleran North America since Jurassic times: double-sided subduction, archipelago collisions, and Baja-BC translation. In review (revision) with GSA Books. Preprint: https://eartharxiv.org/repository/view/7460/