A magma-poor rift model for the Cordilleran margin of western North America

The Cordilleran margin of western North America has an uncertain rift evolution that includes >300 m.y. of lithospheric extension, breakup, and synto post-breakup magmatism. Here I use the Newfoundland-Iberia rift system as a modern analogue to evaluate Cordilleran margin development and propose a magma-poor rift model for western North America. After polyphase Tonian–Ediacaran rifting, early Cambrian breakup resulted in a base-level fall and generation of the basal Sauk I megasequence boundary. A lower to middle Cambrian breakup succession developed over this lithospheric breakup surface and from bottom to top consists of lowstand, transgressive, and highstand systems tract deposits. Lower Cambrian volcanic strata are recognized in proximal breakup successions and predicted to be more voluminous in outboard regions with hyperextended crust and exhumed mantle. Off-axis, post-breakup volcanic strata were generated during the release of in-plane tensile stresses and focusing of extension toward the nascent plate boundary. These findings suggest that ancient magmapoor rift assemblages, including those affected by later convergent tectonism, can preserve their original stratigraphic attributes and be successfully identified in the geological record.