2004 Denver Annual Meeting (November 7–10, 2004)

Paper No. 3
Presentation Time: 2:10 PM


UMHOEFER, Paul J., Department of Geology, Northern Arizona Univ, Box 4099, Flagstaff, AZ 86011, paul.umhoefer@nau.edu

The Baja BC debate has been contentious, in part because the options were wildly different - hypotheses varied between a few 100 km and 3000 km translation. Three new analyses in this session suggest that the debate is more tractable. A new reconstruction (Wyld et al) uses only known fault offsets in the northern Cordillera and reconstructs the S end of Baja BC to the south of its present position adjacent to the southern Klamath Mountains. This hypothesis requires 800 km of translation. An alternative “moderate” reconstruction (Umhoefer, 2003) uses recent paleomagnetic data and plate-scale relations to place the S end of Baja BC 1600 km to the south and along the southern Great Valley Group and southern Sierra Nevada. A third approach (Enkin) is a review of paleomagnetic data and cratonic reference poles that suggests 2100 km translation of Baja BC. This interpretation may be compatible with the moderate translation model if ~500 km of translation occurred from 95 to 85 Ma along Salinia – Mojave. The appeal of the moderate translation model is that it is compatible with new views on the coeval Laramide belt (Saleeby, 2003) and yields a simple plate tectonic scenario. At 85 Ma, Baja BC lay north of the Kula – Farallon – North America triple junction, and the shallowly subducting Farallon plate and Laramide belt lay south of the triple junction; the formation of the Kula – Farallon boundary at 85 Ma led to the profound crustal boundary documented in the southern Sierra Nevada by Saleeby and colleagues. From 85 – 65 Ma, the S end of Baja BC moved rapidly north along the Franciscan in a Kula – North America transpressional belt. Many aspects of California geology are compatible with this translation, but the setting of the Franciscan complex and Great Valley basin must be reinterpreted. The post-65 Ma translation of Baja BC in the moderate-translation model fits the known offsets on strike-slip faults in the northern Cordillera (~800 km), but only if all known faulting was early Cenozoic. This presents a possible solution for Baja BC. Are the paleomagnetic only (2100 km translation), moderate translation (1600 km), and faults only (800 km) models all compatible, but each indicate translation beginning at different times? The resulting scenario suggests 500 km translation of Baja BC from 95 – 85 Ma, 800 km from 85 – 65 Ma, and 800 km from 65 – 40 Ma.