~100 MA WRANGELLIA COMPOSITE TERRANE COLLISIONAL TIMING FROM STRUCTURALLY RESTORING SUBDUCTED SLABS AND MANTLE CONVECTION FORWARD MODELING

Wrangellia composite terrane collisional timing and history is debated, leading to uncertainty regarding its influence on regional orogenesis. Possible times of collision range from Early Jurassic to Late Cretaceous and vary between closure of a single open-ocean back-arc basin to two back-arc basins of differing sizes. We test the timing of this collision by unfolding (i.e., structurally restoring) the upper mantle Cascadia slab below the western US, imaged by mantle tomography, back to Earth’s surface. When reconstructed within a global mantle reference frame, the unfolded Cascadia slab accounts for at least ~75 Ma of Farallon subduction below southern California. This suggests a major slab breakoff event during Late Cretaceous time, that we interpret as a possible time of WCT collision. To test this interpretation, we performed 3D numerical mantle convection forward modeling with imposed surface velocities derived from kinematic plate motion models. We considered several classes of WCT collisional models including: (1) a “Conventional” model involving an Early Jurassic initial collision followed by the opening of a small back-arc basin prior to final suturing in Early Cretaceous time, and (2) a “Late Collision” model constrained by our slab-unfolding results involving the Late Cretaceous closure of a single open-ocean basin. Our modeling results demonstrate that the “Conventional” model reproduces the east-dipping “Mezcalera” lower mantle slab below the North American east coast but fails to reproduce the sub-vertical “Deep Cascadia” slab present at similar depths below the western US. Crucially, the “Late Collision” model succeeds in reproducing both lower mantle slabs, supporting a Late Cretaceous WCT collision around 100 Ma. In this scenario, the Mezcalera slab is linked to east-dipping subduction below western North America. Linking the WCT with the Deep Cascadia slab has profound implications for western North American plate reconstructions including constraining paleo-latitude and longitude for the WCT. The WCT collision may overlap spatiotemporally with the enigmatic Laramide orogeny.