TECTONIC SIGNIFICANCE OF PALEOMAGNETIC RESULTS FROM THE OCHOCO AND HORNBROOK BASINS, OREGON

The Blue Mountains lie immediately west of a large transpressional strike-slip fault zone along which outboard terranes were moved in a dextral sense during Cretaceous time (Wyld and Wright, 2001). To evaluate possible motion of the Blue Mountains, we conducted a paleomagnetic study of the Gable Creek Formation, an Albian-Cenomanian marine clastic sequence deposited on the Blue Mountains "superterrane". Similar marine clastic rocks, the Hornbrook Formation, overlap basement rocks of the Klamath Mts in SW Oregon. 120 samples from 18 sites in the Gable Creek Fm. were thermally demagnetized. All samples displayed two components of remanence. The second-removed (characteristic) component unblocked between 250 and 580 C. The age of the second-removed component is constrained to be mid-Cretaceous by positive baked-contact, conglomerate, and fold tests. The mean direction is D=13.5, I=59.7, a95=5.2, k=71.6 (after tilt-correction). This direction is discordant with the expected mid-Cretaceous direction for North America, indicating a post-Albian clockwise rotation of 37.5 +/- 9.6 degrees and displacement of 1600 +/- 800 km. Prior paleomagnetic studies of late Jurassic plutons in the Blue Mountains indicated 60 degrees of CW rotation of the Blue Mountains since 145 Ma (Wilson and Cox, 1980). The Eocene Clarno Volcanics reveal 16 degrees of post-Eocene CW rotation (Gromme et al., 1986). Combined with our new data it appears that clockwise rotation of the Blue Mountains was ongoing during late Cretaceous time. Mankinen and Irwin (1982) reported paleomagnetic results from two Hornbrook Fm. sites. Combined with results from other Cretaceous rocks (Great Valley ) in N. California, a failed fold test resulted, indicating post-Cretaceous remagnetization of the Hornbrook and no significant post-mid Cretaceous translation or rotation of the Klamath block. The tilt-corrected mean of the two Hornbrook sites is D=21.5, I=54.9, a95=29.5, k=73.7. This direction is very similar to the direction we obtained from the Gable Creek Formation, raising the possibility that the Hornbrook and Mitchell Inlier shared a common tectonic history. If true, significant translation of the Eastern Klamaths block may have occurred during late Cretaceous time. The interpretation of the apparent translation of these rocks will be controversial.