A Paleomagnetic Paleolatitude Determination from the Upper Cretaceous Units of the Gold Beach Terrane, Southwest Coastal Oregon

The paleolatitude for the Gold Beach terrane has been under debate from several sources (Bourgeois, 1980b; Bourgeois, 1985; Jayko and Blake, 1985; Seiders and Blome, 1987; Aalto, 1989) since the 1980 publication of Joanne Bourgeois’ PhD thesis connecting the Upper Cretaceous formations of the Gold Beach terrane to the Nacimiento Block, currently in the vicinity of Southern California; During the Cretaceous, the Nacimiento Block was farther south than its present position by about 200 km (Blake and Jayko, 1985). The opposing hypothesis of Seiders and Blome (1987) connects the Gold Beach terrane to the neighboring Klamath block, implying at most 300 km of northwards transport for the Gold Beach terrane (Seiders and Blome, 1987). Paleomagnetic study of the Upper Cretaceous units of the Gold Beach terrane has determined that the units originated south of their present position by 1550 +/- 500 km, at a latitude of 34° during the Campanian, placing the origin of the Upper Cretaceous units at the Cretaceous latitude of northern Baja, Mexico. A compaction correction was applied using the results of AMS-determined rock magnetic fabric, showing a small amount of post-Cretaceous inclination shallowing. After correction for tectonic rotation, AMS fabric implies an eastward derived sediment source, possibly from the Nacimiento Block. These results substantiate the hypothesis of Bourgeois (1980), Blake and Jayko (1985) for a southerly latitude of origin of the Gold Beach terrane and rule out the hypothesis of Seiders and Blome (1987) for a local origin connected to the Klamath block. The units are overlain by the Mio-Pliocene Empire Formation at Blacklock Point, however several neighboring late Cretaceous terranes are overlain by the mid- Eocene Lookingglass Formation, capping the emplacement age of the Gold Beach terrane to about 50 Ma. The displacement estimated by this study compares favorably to plate motion rates derived by Engebretson et al., (1984) for translation of this terrane on the Kula plate to its present position by 50 Ma