Paleomagnetism of the Upper Jurassic Galice Formation, southwestern Oregon: Evidence for differential rotation of the eastern and western Klamath Mountains

Abstract

Paleomagnetic studies of low-grade metatuff within the Upper Jurassic Galice Formation at the type section along the Rogue River, Oregon, indicate stable magnetizations at 13 sites with a mean orientation of D = 245.6°, I = 55.9°, and α 95 of 5.2°. Site mean directions are well grouped in situ despite the variable orientation of both bedding and cleavage. The magnetizations are therefore secondary and postdate Late Jurassic folding. The most likely times for remagnetization are during low-grade metamorphism of the Late Jurassic Nevadan orogeny (150 Ma) or during intrusion of the nearby Grants Pass pluton (139 Ma). The paleomagnetic pole position calculated from the formation mean direction lies at 40°N, 31°E, far from North American cratonic pole positions aged 150 Ma and younger. Clockwise rotation of the study area by 99°±10° and tilting or poleward displacement of 0°±7° are indicated by comparison with a reference pole aged 150 Ma. However, the amount of rotation is not significantly different for other reference poles aged 140 to 80 Ma. Thus, the data indicate that the Galice stratotype and vicinity have rotated approximately 100° since Late Jurassic time, 20° to 40° more than the eastern Klamath Mountains. An unrecognized fault may be required to accommodate the difference in rotation. In addition, restoration of the rotation places Nevadan structural trends in the western Klamaths at a high angle to Nevadan trends in the nearby Sierra Nevada and Blue Mountains of Oregon.