TOMOTECTONIC CONSTRAINTS ON ASSEMBLY OF CORDILLERAN NORTH AMERICA SINCE PANGEA BREAKUP (Invited Presentation)

Quantitative plate reconstructions rely on seafloor isochrons, but constant recycling of lithosphere ensures that only ~50% of seafloor predates Cenozoic, and almost none is older than Jurassic. Isochron subduction hampers efforts to reconstruct Cordilleran North America, but the lithosphere is not destroyed; it persists within the mantle as folding slabs, cooler than ambient mantle and therefore visible to seismic tomography.

Our tomotectonic analysis uses subducted slabs beneath North America, highly resolved thanks to USArray, in combination with surviving isochrons, to reveal evolving arc, trench, and plate geometries back to Pangea breakup. Land geological evidence serves to independently validate or reject the inferred paleogeographies. This addresses fundamental questions about the assembly of North America, e.g., did an Andean-type margin exist along western North America for all times between Cenozoic Cascade arc formation and the Jurassic, when an arc was rooted in continental crust of southwest USA?

Not according to deep mantle and seafloor isochron evidence. Instead, TWO massive arc complexes originated in the seas west of Pangea during Pangea’s early fragmentation (~190-170 Ma), a time when eastward subduction beneath the continental margin arc was shutting down. A >10,000 km long, east-pointing chevron of slab walls in the lower mantle, with its apex near Nova Scotia today, marks paleo-arc locations 2000-4000 km off the west coast of Pangea. The slab’s westerly location and geometry are not consistent with continent-hugging, Jurassic to Recent Farallon subduction. Instead, the second massive and more westerly slab wall (4000 – 6000 km W of Pangea) must have received all northern Farallon lithosphere, and continues to do so beneath the Cascades. Both slabs initiated intraoceanic. Their arcs did not contribute to an Andean-style margin until they were diachronously overridden by North America, beginning ~155 Ma. Implied is a continent-spanning suture between these two arc complexes and North America. We demonstrate evidence of this suture in an Alaska to Mexico tract of at least 12 Jura-Cretaceous basins (over half of which contain relicts of mantle) that collapsed between the arc complexes and the continental margin. An analogous suture is forming today as Australia overrides arcs to its north.