MODERATE (1600 KM) NORTHWARD TRANSLATION OF BAJA BRITISH COLUMBIA FROM SOUTHERN CALIFORNIA: AN ATTEMPT AT RECONCILIATION OF PALEOMAGNETISM AND THE RELATION OF BAJA BC TO THE LARAMIDE

We present a moderate translation model of 1600 km of Baja BC based on a compilation of paleomagnetic data (Enkin, 2006), and a plate tectonic configuration for Baja BC and the Laramide orogeny. We began with a 100 Ma map of the Cordillera with all major strike-slip faults restored. We then added offsets of ~700 km and ~500 km along major faults in order to match the translation of 1600 km for Baja BC and 1000 km for the Intermontane superterrane. Baja BC moves north at 32 – 40 km/m.y. along the Kula – North America plate boundary. The southern end of Baja BC began its northward translation at 85 Ma west of the southern Sierra Nevada and was adjacent to the Klamath Mountains at 65 Ma. There are many aspects of the geology of California that are compatible with the model. The post-65 Ma translation of Baja BC fits the known offsets on strike-slip faults in the northern Cordillera. Our model fits with two scenarios for the mid Cretaceous before 85 Ma (i) Wrangellia and related terranes of Baja BC accreted to North America in the Jurassic at the latitude of Oregon, moved south ~800 km in the Early Cretaceous, and moved 1600 km with Baja BC to their present position. Alternatively, the Wrangellia – Alexander arc system was offshore North America until accretion at 95 – 85 Ma along California to British Columbia, followed by northward translation. Our model fits well with interpretations of the Laramide orogeny (shallow slab) being restricted to a ~500-km wide corridor from the Salinian - Mojave region to the NE toward the Colorado Plateau and Wyoming. The profound change in the geology of the Cretaceous magmatic belt from Sierra Nevada to Mojave in the latest Cretaceous is here explained by the formation of a new triple junction outboard of the southern Sierra Nevada. This model does not preclude the expansion of the “Laramide” region to the north as Baja BC moved rapidly northward. We present predictions for matches of basins and source regions from this model.