Tomotectonics of Cordilleran North America since Jurassic times: double-sided subduction, archipelago collisions, and Baja-BC translation

Karin Sigloch , Mitchell G. Mihalynuk

 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. Tomotectonics uses land geological observations for validation. We explain the tomotectonic method, with a conceptual separation of its (geophysical) hypothesis-generating stage from its (geological) hypothesis-testing stage.  

We generate a full suite of tomotectonic inferences for the North American Cordillera from the slab assemblage now occupying the mantle to depths of 1800-2000 km. We reason why this assemblage originated as a completely intra-oceanic archipelago, at a time of worldwide tectonic reorganization around 200-170 Ma, when the Atlantic began to spread, and the Pacific plate was born. The Archipelago was bounded by two sets of trenches that pulled in seafloor from the east/northeast and from the southwest. On the archipelago’s western boundary, purely oceanic plate was consumed beneath intra-oceanic Farallon arcs; beneath the eastern boundary, ocean lithosphere attached to western North America was consumed, until none remained. The resulting collision between the continental margin and the eastern bounding arc (which was built on Insular Superterrane) was diachronous, commencing ~155 Ma (Nevadan orogeny), and continuing through the Sevier orogeny times. Upon collision, subduction polarity was forced to flip at the affected latitudes, so the pre-existing Farallon trench grew southward to accommodate the new kinematic requirement for eastward subduction. The new southern Farallon subduction produced arcs on the continental-margin, including the Sierra Nevada Batholith ~120-80 Ma. 

Slab-free areas hindcast that this Andean-type margin at U.S. latitudes did not persist beyond ~80 Ma, and that it must have been followed by a dextral transform regime, i.e., by boundary conditions conducive to large-scale terrane translations. This transform regime ended when the northern Farallon arc, still sitting offshore until ~75-50 Ma gradually collided with the margin to become the continental Cascades arc. In the northeastern boundary arcs of future Central Alaska collided with the continental margin 90-50 Ma, and then gradually collided with the northern Farallon arc. Oblique override of these two oceanic arcs produced a range of collision styles, including the Baja-BC northward sprint and the assembling of Alaska. From its combined accounting of arc override events over the past 170 Ma, tomotectonics indicates large-scale northward displacement of Insular Superterrane since its accretion, in direct and independent support for the “Baja-BC” hypothesis of paleomagnetism.