Paleomagnetic evidence for tectonic rotation of Jurassic plutons in Blue Mountains, eastern Oregon

Abstract

To test the hypothesis that pre‐Tertiary rocks of eastern Oregon have undergone clockwise rotation, a paleomagnetic investigation was made of granitic and high‐temperature metamorphic rocks of the upper Jurassic‐Lower Cretaceous intrusive suite of the Blue Mountains province, northeastern Oregon. At nine sampling sites in three widely separated regions the paleomagnetic directions showed a high level of intrasite and intersite consistency, with a mean direction of I = 63°, D = 30°, a95 = 10°. The observed inclination is consistent with that expected on the basis of paleomagnetic results for stable North America, indicating that the intrusives have not undergone northward translation since cooling. The paleomagnetic declination, on the other hand, indicates that the region sampled has undergone a clockwise rotation of 60°±29° relative to the stable craton. Two of the bodies sampled are intrusive into the exotic Seven Devils terrane, and the remaining body is intrusive into the tectonic melange of a separate arc‐trench terrane, which may well have been the North American margin. The consistency in magnetic directions between the three regions (k = 151) indicates that suturing of the exotic terrane took place prior to the emplacement of the intrusives. The early Tertiary Clarno Formation situated to the west of the region sampled has rotated substantially less than the Blue Mountains [Beck et al., 1978], indicating that much of the rotation of the Blue Mountains occurred prior to the Eocene. To account for the observed rotations, we discuss two alternative models. In the first, rotation accompanies back arc rifting associated with the Early Cretaceous westward rifting of the Klamath Mountains. In the second, rotation reflects Late Cretaceous regional dextral shearing in the Blue Mountain area, which is located several hundred kilometers south of major right lateral faults known to extend from Alaska down at least to northern Washington.