ROTATION OF THE OREGON COAST RANGE: A CAUTIONARY TALE

Greater than 45 degrees of clockwise rotation of the Oregon Coastal Block over the last 50 m.y. has been widely accepted for many decades. However, much of the paleomagnetic evidence from Eocene sediments is outdated and no longer meets modern analytical standards (e.g. Simpson and Cox, 1977). Here we present new paleomagnetic data from the Eocene Tyee Formation of the central Oregon Coast Range. Results from over 600 paleomagnetic cores show only modern directions. Most, but not all, sites are co-located with sites described in detail in the Ph.D. thesis of Simpson (1977). Furthermore, a fold test performed across multiple sites fails. This shows that the sediments of the Tyee Fm. do not carry a primary remanent magnetization, and therefore cannot support a rotational history for the Oregon Coast Range. Our results do not exclude the possibility that the Coast Range has rotated. Instead, they show that much of the initial data used to infer large magnitude rotation since Siletzia accretion are not reproducible.