Cordilleran Section - 115th Annual Meeting - 2019

Paper No. 32-5
Presentation Time: 9:25 AM


UMHOEFER, Paul J., School of Earth and Sustainability, Northern Arizona University, Flagstaff, AZ 86011

In 2006, Umhoefer and Blakey proposed that Baja British Columbia (BC) was translated 1600 km northward from 90 – 40 Ma based on a new analysis of paleomagnetic data by Enkin. That translation placed southern Baja BC west of the Franciscan belt and Sierra Nevada and immediately north of the Mojave segment of the Cretaceous batholith, and thus it was compatible with the Laramide flat slab interpretation of Saleeby. In this Baja BC model, the Farallon – Kula spreading ridge formed off southern California at ~90 Ma and Kula – North America (NAM) oblique convergence was the driver of the translation. A summary by Wyld et al. of known strike-slip faults of the northern Cordillera suggested that at a minimum SW Baja BC (NW Washington) lay along the Klamath Mountains near the end of the Cretaceous. A series of recent studies of basin strata and metamorphic rocks using detrital zircons and isotopes collectively suggest that the moderate Baja BC (Mojave BC) hypothesis should be revisited. Known fault offsets across Baja BC from 65 - ~40 Ma of 450 – 600 km suggest 15 – 25 km/my overall rates of translation; the residual 1000 – 1150 km of translation implied for the Late Cretaceous gives a rate of translation of about 40 km/my. These rates are a reasonable fraction of the rapid 100 - 150 km/my for Kula – NAM relative motion in the Late Cretaceous through Eocene from global plate models; only part of this large translation (40-60%) has been identified with known fault offsets. The “Mojave BC” model would predict these affects from rapid oblique convergence during the Late Cretaceous at the latitude of California and Nevada: (i) a Coast Mountains-North Cascades arc and forearc basins with the Chugach complex to the west; (ii) a Franciscan transpressional belt supported by strike-slip faults of Late Cretaceous-Paleogene age; (iii) the Great Valley Group transitioning from a forearc to transpressional basin; (iv) shut off of the Sierra Nevada arc because of highly oblique convergence and a possible flat slab from the young ocean crust south of the Farallon - Kula ridge; (v) a possible slab gap evolving to flat slab under the Nevadaplano. The Paleogene translation of the Mojave BC block in the Pacific Northwest is complex, but is compatible with the formation and collision of the offshore Siletzia oceanic plateau, and the Eocene history of the North Cascades.