REVISITING ROTATION OF THE OREGON COAST RANGE

It is widely accepted that the Oregon Coastal Block has experienced clockwise rotation of ~60 degrees over the last 50 m.y. However, the paleomagnetic evidence for this is outdated and does not meet modern standards (e.g. Simpson and Cox, 1977). In this study, we present new paleomagnetic data from the Eocene Tyee Formation of the central Oregon Coast Range. Of the 580 paleomagnetic cores that were measured in this study from 10 different localities, ~50 samples (8.6%) were consistent with the clockwise rotation hypothesis. The remaining ~530 samples show directions similar to the modern axial dipole or preserve a different remanence direction. No reversals were found within the Tyee, and a fold test yielded inconclusive results due to the lack of reliable samples. Overall, our results suggest that the original paleomagnetic evidence for the rotation hypothesis is not reproducible. Our results do not exclude the possibility that the block has rotated since Siletzia accretion, but paleomagnetic evidence for large rotations will need to be reevaluated in other formations within the Oregon Coast Range.