RECONNAISSANCE PALEOMAGNETIC INVESTIGATION OF WALLOWA TERRANE VOLCANICS NEAR WHITE BIRD, IDAHO, SUGGEST 16-30 DEGREES NET CLOCKWISE ROTATION AFTER 200 MA

Allochthonous Permian to Jurassic rocks of the Wallowa arc terrane and related terranes of the Blue Mountains province are juxtaposed against autochthonous rocks of North America along the Salmon River suture in west-central Idaho. Rotation and translation of allochthonous terranes farther west has been well documented with paleomagnetism of Cretaceous and Cenozoic rocks, with some attributed to dextral shear during oblique convergence. However, paleomagnetic quantification of the complex histories of older rocks of terranes such as the Wallowa has been difficult because of deformation and metamorphism of layered rocks and lack of paleohorizontal for plutonic rocks.

We report here results from 11 sites in young rocks of the Wallowa terrane between the lower Salmon and the Snake rivers that have experienced less metamorphism and remagnetization than prevously studied older rocks to the southwest. A conglomerate test from the red tuff unit near Pittsburg Landing, Idaho, and a reversal test using all 8 acceptable sites, are consistent with the shallowest magnetic components being primary. A fold test is equivocal in that it rules out remagnetization in both the rocks' present attitudes and when corrected to assumed paleohorizontal. We interpret the latter to indicate directions were overcorrected for unrecognized original dip, so provide results both fully and partially tilt corrected. A U-Pb zircon age of 197 Ma (Tumpane, 2010; Northrup et al., 2011) from a flow we sampled in the red tuff unit suggests it was magnetized about 200 Ma or a bit later.

Rotation and poleward translation derived from the overall mean direction are slightly dependent on choice of tilt correction but highly dependent on choice of reference pole. Using the 200 Ma J1 cusp of an Euler apparent polar wander path implies significant clockwise rotation of 20° and 16° for full and half tilt corrected directions but no poleward translation. Using a 201 Ma paleomagnetic pole from eastern North America requires clockwise rotation of 30° or 25° and considerable northward translation. Note that these are net changes and that results from younger rocks as well as better understanding of structural history are needed to resolve separate events; insignificant rotation or transition could be the sum of equal but opposite motions.