CRETACEOUS DEXTRAL STRIKE-SLIP FAULTING IN THE NORTH AMERICAN CORDILLERA: WHERE DO WE GO FROM HERE?

Recent and ongoing research by many workers indicates that dextral strike-slip faulting in the N. American Cordillera was widespread in both the Early and Late Cretaceous, and resulted in significant margin-parallel terrane displacements. These relations lead to several interesting questions whose possible answers form the focus of this talk. (1) What is the plate tectonic setting and cause of dextral motion? Possibilities are strain partitioning due to oblique subduction, a transform plate margin, or escape tectonics following major collision. Plate tectonic reconstructions based on paleomagnetic data support oblique subduction as the cause for Late Cretaceous dextral offset, but are less revealing for the Early Cretaceous. Geologic relations indicating a pronounded regional decline in Cordilleran arc magmatism between 140-120 Ma, however, provide insight into tectonics and suggest that the plate boundary may have been dominated by transform motion at this time. If so, it begs the question of what drove this change (ridge subduction?). (2) How can we reconcile the distribution of Late Cretaceous strike-slip faults in the NW Cordillera with those farther south, and what happens to Early Cretaceous faults of the U.S. Cordillera north of west-central Idaho? We speculate on some possibilities, but this is a topic that needs much more study. (3) Are there other, as yet unidentified, major Cretaceous strike-slip boundaries in the Cordillera? Probably not in western Canada and NW Washington, where Cretaceous strike-slip faults have been recognized for many years, but the situation farther south is not so established. We speculate that some previously enigmatic structural boundaries and features in the U.S. Cordillera may reflect unrecognized Cretaceous strike-slip offset. (4) How do reconstructions of Cretaceous strike-slip faults affect interpretations of older geologic relations? We emphasize one particularly obvious example where re-analysis is needed. Using a reconstructed map, Jurassic oceanic arc assemblages of the Klamath Mountains and western Sierra Nevada restore south to a position directly west of coeval continental arc assemblages of the southwest Cordillera. This relation appears to require that an ocean basin originally separated these arcs and that they did not evolve on the same plate.