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Crustal blocks accreted to North America form two major belts that are separated by a tract of collapsed Jurassic–Cretaceous basins extending from Alaska to Mexico. Evidence of oceanic lithosphere that once underlay these basins is rare at Earth’s surface. Most of the lithosphere was subducted, which accounts for the general difficulty of reconstructing oceanic regions from surface evidence. However, this seafloor was not destroyed; it remains in the mantle beneath North America and is visible to seismic tomography, revealing configurations of arc-trench positions back to the breakup of Pangea. The double uncertainty of where trenches ran and how subducting lithosphere deformed while sinking in the mantle is surmountable, owing to the presence of a special-case slab geometry. Wall-like, linear slab belts exceeding 10,000 km in length appear to trace out intra-oceanic subduction zones that were stationary over tens of millions of years, and beneath which lithosphere sank almost vertically. This hypothesis sets up an absolute lower-mantle reference frame. Combined with a complete Atlantic spreading record that positions paleo–North America in this reference frame, the slab geometries permit detailed predictions of where and when ocean basins at the leading edge of westwarddrifting North America were subducted, how intra-oceanic subduction zones were overridden, and how their associated arcs and basement terranes were sutured to the continent. An unconventional paleogeography is predicted in which midto late Mesozoic arcs grew in a long-lived archipelago located 2000–4000 km west of Pangean North America (while also consistent with the conventional view of a continental arc in early Mesozoic times). The Farallon Ocean subducted beneath the outboard (western) edge of the archipelago, whereas North America converged on the archipelago by westward subduction of an intervening, major ocean, the Mezcalera-Angayucham Ocean. The most conspicuous geologic prediction is that of an oceanic suture that must run along the entire western margin of North America.  It formed diachronously between ca.  155  Ma and ca.  50  Ma, analogous to diachro nous suturing of southwest Pacific arcs to the northward-migrating Australian continent today. We proceed to demonstrate that this suture prediction fits the spatio-temporal evidence for the collapse of at least 11 Middle Jurassic to Late Cretaceous basins wedged between the Intermontane and Insular-Guerrero superterranes, about half of which are known to contain mantle rocks. These relatively late suturing ages run counter to the Middle Jurassic or older timing required and asserted by the prevailing, Andean-analogue model for the North American Cordillera. We show that the arguments against late suturing are controvertible, and we present multiple lines of direct evidence for late suturing, consistent with geophysical observations. We refer to our close integration of surface and subsurface evidence from geology and geophysics as “tomotectonic analysis.” This type of analysis provides a stringent test for currently accepted tectonic models and offers a blueprint for similar, continental-scale investigations in other accretionary orogens.

Mantle and geological evidence for a Late Jurassic−Cretaceous suture spanning North America