INDIA-LIKE COLLISION OF MASSIVE ARC CHEVRON CAUSED NEVADAN OROGENY

High resolution mantle tomography images a lithospheric graveyard beneath North America. Between ~800 and ~2000km depth, two massive vertical walls form a 10 000 km-long east-pointing chevron. These walls accumulated during westward subduction beneath adjacent arcs that remained stationary within the mantle reference frame. Hence the slab walls indicate the absolute locations of an intra-oceanic, massive arc chevron (MAC) during initial Atlantic Ocean opening. Continent reconstruction in a hotspot reference frame places the linear western coast of Ancestral North America (ANA) at a distance of >2000 km east of the MAC. Forming above the northern wall were Jura-Cretaceous arcs that, together with their substrates, crumpled during translation to form western Alaska; the arc above the southern slab wall nucleated upon the Insular Superterrane (INS), a microcontinent similar in length the Indian subcontinent.

Diachronous collision of the MAC began at its INS-cored apex, as recorded by early deformation (~160-155 Ma) in the southern Canadian Rockies and Intermontane belt plus allied arcs in the US that were loosely accreted to the continental margin by Early Jurassic times. MAC geometry persisted for ~30 m.y. as the apex was driven into ANA. Analogous to the Tethyan Himalayan sequence, thick accumulations of flyschoid sediments trapped between MAC and advancing ANA are preserved as Middle Jurassic to Early Cretaceous successions from Alaska to southern California. Drastic upward flattening of the vertical wall at the MAC apex started ~130 Ma, signaling a transition from collisional shortening of the margin to forced eastward subduction of Farallon lithosphere and migration of the trench with the ANA margin: the end of Nevadan deformation. South and north of the collapsed MAC apex, oceanic crust continued to subduct westward until late Early Cretaceous (Paleocene in the far north).

This tectonic scenario, now constrained quantitatively by modern tomography and plate reconstructions, is similar to those proposed more than four decades ago by Moores (1970) and later workers (e.g. Schweickert and Cowan, 1975; Ingersoll and Schweickert, 1986).