ASSESSMENT OF INCLINATION SHALLOWING IN OCHOCO BASIN, MITCHELL INLIER, CENTRAL OREGON

Cretaceous paleogeographic reconstructions of the Blue Mountains province (BMP) in central Oregon propose variable magnitudes, and timing, of the northward translation of these terranes along the Cordillera. Some of the models that propose significant northward displacement (e.g. Hildebrand et al., 2015) rely on paleomagnetic data from clastic, sedimentary rocks (Housen and Dorsey, 2005). This study investigates the potential extent of inclination shallowing in Cretaceous sedimentary rocks of Mitchell Inlier, which record the Mid-Cretaceous paleolatitude of the BMP. This work will be combined with provenance studies, U-Pb and Hf analysis of detrital zircons from the same strata in Mitchell Inlier (Gulliver et al., 2016; Patzke et al., 2016).

A total of 168 specimens have been collected from 17 sites within the Gable Creek and Hudspeth formations of Mitchell Inlier. Alternating field and thermal demagnetization has been carried out on 10 sites (98 specimens), with 9 sites and 83 specimens yielding well defined characteristic remanent magnetizations (CHRM). After applying bedding corrections, these sites yield a mean direction of D = 27.9°, I = 61.8°, k = 186.5, α₉₅ = 3.8° and N = 9. Previous paleomagnetic work on rocks of Mitchell Inlier by Housen and Dorsey (2005) is consistent with this direction. Combining the site mean directions of this study and those of Housen and Dorsey (2005) yields a mean magnetic direction of D = 20.8°, I = 63.5°, k = 33.6, α₉₅= 5.7° and N = 20.

Anisotropy of magnetic susceptibility (AMS) ellipsoids are mostly oblate, but have maximum and intermediate axes that are tightly clustered in the bedding plane. This symmetry may reflect either sediment transport processes (e.g. Novak et al., 2014) or weakly developed deformation fabrics. Further analysis of magnetic fabrics, and the use of magnetic anisotropy to evaluate inclination error is underway. Trial inclination error corrections were applied to our specimens (N = 137) using the Elongation-Inclination (E/I) method. Initial results yield a corrected inclination of 71.1° +7.9°/-10.1°, suggesting 7.6° of shallowing and a corrected paleolatitude of 55.6° +13.4°/-13.6°. Future work will focus on obtaining a larger specimen population to decrease the 95% confidence bounds of these calculations.