PALEOMAGNETIC RESULTS FROM CRETACEOUS CLASTIC SEDIMENTS: POSSIBLE IMPLICATIONS FOR THE PALEOGEOGRAPHY OF THE BLUE MOUNTAINS, KLAMATHS, GREAT VALLEY ROCKS

The majority of paleogeographic models for Cordilleran terranes have, until recently, proposed that the collage of terranes extending from the Blue Mountains (E Oregon) to the Klamaths (SW Oregon, NW California) have remained more or less in place for the past ~130 Ma. A set of recent models (Wyld and Wright, 2001, Wyld et al, 2007), based primarily on geological correlations, has proposed that these terranes were located 300 to 700 km to the south at the beginning of the Cretaceous (~140 Ma). Independent tests of these models can be made by use of new, and existing, paleomagnetic data from these terranes. Work by Mankinen and Irwin (1982) on Cretaceous sediments from the Klamaths and the northern Great Valley group found that most of these rocks have been remagnetized, based on a regional tilt-test among these units. A significant subset of these rocks were found, however, to retain a pre-folding magnetization. Included in their study are two sites from the Hornbrook Formation. Work by Housen and Dorsey (2005) found that the Cretaceous sediments of the Mitchell Inlier (Blue Mountains) retained a primary magnetization. Close comparison between the paleomagnetic results from the Hornbrook and Mitchell Inlier rocks indicates that both units share a similar mean paleomagnetic direction in tilt-correct coordinates. Taken together, these paleomagnetic results indicate that the Blue Mountains, Klamaths, and the northern portion of the Great Valley group rocks were located at a paleolatitude of 40 N, approximately 1100 to 1700 km south of their present location wrt to North America. These paleomagnetic displacement estimates are somewhat greater than those estimated by Wyld and Wright (2001 and Wyld et al 2007), and also made using rocks of 110 to 90 Ma. Possible explanations for these differences include: 1) sinistral motion of these terranes between 140 and ~100 Ma, 2) adjustment of the tectonic models to also account for the 43 N mid-Cretaceous paleolatitude of the Sierra Nevada, also based on paleomagnetic data, or 3) undetected inclination error in the Cretaceous sediments.