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

Paper No. 7
Presentation Time: 8:00 AM-12:00 PM


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

A related abstract in this session describes the rationale for the moderate translation (1600 km) of Baja BC from 85 to 40 Ma. In this poster, we will present an 85 Ma reconstruction that shows the entire Cordillera from Mexico to Alaska, and a series of large format maps of the 1600-km-translation reconstruction with an emphasis on the relations at the latitudes of the western United States. These maps will provide a forum for discussion and analysis and allow us to explore several aspects of the reconstruction. (1) The reconstruction allows us to ask what fragments now to the north may have been the missing outer convergent margin of the Salinian – Mojave block and the outer parts of the truncated southern Great Valley Group? Candidates appear to be the Ochoa and Methow basins. (2) We can relate the maps to the timing and type of events that occurred at the S end of Baja BC and along the corridor between the Salinian - Mojave block and the Sierra Nevada. We will show a specific plate tectonic scenario that predicts a series of testable events. (3) The model demands that the Franciscan was a transpressional belt and the Great Valley Group a transpressional or transtensional basin from 85 to 65 Ma. We can explore where and when this must occur along the California margin in detail. A transtensional setting for the Great Valley basin may be preferred as it remained a largely marine basin in the Late Cretaceous, while Baja BC to the west of California had only terrestrial or no deposition after 85 Ma, which suggests broad uplift across Baja BC. (4) We’ll explore how changes in the interior of the western U.S. may reflect the Kula - Farallon boundary moving rapidly north from southern California to Seattle between 85 and 45 Ma. If the part of the Farallon plate that formed in the wake of the north-moving plate boundary was a normal subducting slab, then the Farallon plate would have a major segment boundary off southern California. (5) The model suggests that 800 km of faulting is presently not accounted for along and east of Baja BC. To focus future study, we will show where this faulting is likely to be.