CRETACEOUS COLLISION RECONCILES WESTERN NORTH AMERICAN TECTONICS WITH DEEP MANTLE SLABS (Invited Presentation)

We present a plate tectonic reconstruction of the North America Cordillera since Mesozoic times using a paired “backward-forward” modeling approach using structural unfolding of subducted slabs from seismic tomography and forward geodynamic models. The southernmost Cascadia slabs account for east-dipping Farallon subduction below south and central California back to the Late Cretaceous 80 to 100 Ma. Prior to 100 Ma, we reconstruct concurrent east-dipping intra-oceanic subduction of the Farallon plate under the Wrangellia Composite Terrane (WCT) and east-dipping subduction of a marginal sea beneath western North America. The intra-oceanic subduction terminated around the mid-Cretaceous (100 ± 7 Ma) due to WCT collision with western North American north of Baja California. We reconstruct significant (>2,000 km) post-100 Ma northward terrane translation of the WCT based on the latitudinal extent of lower mantle slabs under the present Cascadia subduction zone (i.e. Deep Cascadia slabs). Forward geodynamic models show that WCT collision with North America during the Cretaceous reproduces observed mantle structure whereas an earlier Jurassic collision does not. We compare our reconstruction against a new restoration of Cordilleran strike-slip faults, surface geology, and WCT paleomagnetic data. Our reconstructed WCT collision and northward translation is consistent with transpressional shear zone activity in southern California through SE Alaska.