PALEOMAGNETISM OF THE BLUE MOUNTAINS, OREGON, AND RELATED CORDILLERAN TERRANES

An important element in reconstructions of the Cordilleran margin of North America include longstanding ideas regarding the timing and amount of rotation of the Blue Mountains in eastern Oregon. Paleomagnetic results from late J-early K plutonic rocks (Wilson and Cox, 1980), and secondary directions from Permo-Triassic units of the Wallowa-Seven Devils arc (Hillhouse et al, 1982, Harbert et al, 1995) indicate ~60° CW rotation of the Blue Mtns. in accord with prior tectonic models (i.e., Dickinson and Thayer, 1978). 

Applications of geomagnetic field geometry recognize a latitudinally-dependent variation in the shape of paleomagnetic directional dispersions (Tauxe et al, 2008). These same predicted shape dispersions can also be used (Housen, 2014) to evaluate tilt in rocks that lack paleohorizontal such as plutons and remagnetized strata. 

Mesozoic sedimentary rocks of the Suplee-Izee area (Dickinson and Vigrass, 1965), are remagnetized and have magnetizations that match those of late J-early K plutons. Directions from 64 sites of these rocks yields a mean of D = 33°, I = 64°, k= 26, α₉₅ = 3.7°. For the Blue Mountains rocks, the optimal E-I relationship yields a corrected inclination of I = 65° (+7°/-4°), and estimated paleolatitude of 47°N. Comparing the directions with NA reference poles, a CW rotation of 57 +/- 11 degrees with translation of 1100 +/- 700 km is found. 

A related example is from the Cretaceous Sierra Nevada batholith (Hillhouse and Grommé, 2011). Directions from 94 samples have a mean of D = 340°, I=68°, k=49, α₉₅ = 2.1°. The E-I relationship suggests a corrected mean inclination of I=68° (+8°/-3°), and estimated paleolatitude of 51°N.

Together with data from Cretaceous and Eocene rocks, 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, and this is also consistent with ~1100 km of northward translation during some of this time. The SNB results indicate no translation of this key tectonic unit, and the E/I results do not indicate tilt, or deformation. This, as well as other paleomagnetic data, cast doubt on recent models (Hildebrand, 2015) calling for large, en-bloc translation of much of the NA Cordillera in post-Cretaceous time.