PALEOMAGNETISM OF MESOZOIC-CENOZOIC ROCKS OF THE BLUE MOUNTAINS, OREGON

A number of existing paleomagnetic studies of Mesozoic and Cenozoic rocks from the Blue Mountains, along with new results, can be used to address the tectonic history of this region. Demagnetization experiments of 3 sites from the Bald Mountain Batholith, 2 sites from small intrusive bodies near Ritter, OR, and 6 sites from the Wallowa Batholith yielded well-defined magnetization components essentially identical to the results of Wilson and Cox, 1980. The combined mean direction of both sets of data from these late Jurassic to early Cretaceous intrusive rocks is D = 30, I = 63, a95 = 6. Samples from Jurassic sedimentary rocks in the Suplee-Izee area include 4 sites of the Lonesome Fm, 3sites of andesitic volcanics in the Snowshoe Fm, and 3 sites from the Trowbridge Fm. The Lonesome and Trowbridge samples all had very well-defined, two component magnetizations. The in-situ mean of the combined Lonesome and Trowbridge Fms is D = 28, I = 63, a95 = 15. Upon tilt-correction, the site means of these units scatter and fail the paleomagnetic fold test in spectacular fashion. The similarity between the directions obtained from the remagnetized Jurassic rocks, and from the late J to early K plutonic rocks suggests that a widespread remagnetization accompanied emplacement of the intrusives. Comparing the directions of the remagnetized Jurassic strata and the late J-early K intrusives with NA reference poles, a CW rotation of 71 +/- 11 degrees with no translation is found using a late J NA pole, and a CW rotation of 57 +/- 11 degrees with translation of 1100 +/- 700 km is found using an early K NA pole. Paleomagnetic studies of ~95-100 Ma sedimentary rocks of the Mitchell Inlier (Housen and Dorsey, 2005) indicate 37 +/- 5 degrees of CW rotation and 1700 +/- 500 km of northward translation. Paleomagnetic results from the 45 Ma Clarno volcanics (Gromme et al., 1986) indicate 16 +/- 10 degrees of CW rotation with no latitudinal translation. All the data suggest that CW rotation of the Blue Mountains has occurred throughout the past 140 Ma, with long-term rotation rates of 0.4 to 1 degree/Ma. The long-term record of CW rotation suggests that the Blue Mtns have experienced dextral shear during from ~140 to ~50 Ma, and is also consistent with the ~1500 km of translation obtained by paleomagnetic data.