ASSESSMENT OF INCLINATION SHALLOWING IN UPPER-CRETACEOUS TURBIDITES, OCHOCO BASIN, CENTRAL OREGON: IMPLICATIONS FOR CORDILLERAN TERRANE TRANSPORT

The North American Cordillera is a complex collage of allocthonous terranes, which record a history of continental growth and deformation. Accurate paleogeographic reconstructions of these terranes are crucial in efforts to produce a comprehensive model for the tectonic evolution of the Cordillera. Some geologically observed displacements across fault zones suggest significantly less translation than inferred from paleomagnetic paleolatitudes. These discrepancies have been attributed to a lack of paleohorizontal control in igneous rocks and anisotropic processes associated with the acquisition of remanence in sedimentary rocks, which may bias magnetic directions toward lower inclinations (Butler et al., 2001). The goal of this study is to apply inclination corrections to Upper-Cretaceous sedimentary rocks of the Hudspeth and Gable Creek Formations. Magnetic directions recorded in these rocks comprise the Late-Cretaceous paleopole of the Blue Mountains Province, establishing its paleolatitude at this time.

Alternating field and thermal demagnetization were applied to samples from 37 sites. Second removed components were isolated at treatments above 30 mT or 300 °C and up to ~80 mT or ~450-585 °C, with a site-mean average of D=22.0°, I=61.8°, k =128.7, α₉₅=2.2°, N=33. Evaluation and correction for inclination shallowing can be accomplished by applying a flattening factor to a distribution of directions and comparing their elongation and mean inclination (E/I) to model curves (Tauxe and Kent, 2004). The E/I method was applied to 463 well-defined directions, correcting their mean inclination to 71.8 +6.2°/-4.8°, or 10° of shallowing. A second method relates the bulk magnetic anisotropy to biases in an orientation distribution of magnetic particles. Using full-tensor fabric corrections (Kim and Kodama, 2004), magnetic directions are steepened by a mean of 4° and a maximum of 8.5°. Because some site fabrics are likely composite fabrics, influenced by sediment transport processes and compaction, this method is considered a minimum estimate of shallowing. Using the fabric-based correction and comparing with the location of North America during Late-Cretaceous time, a paleolatitude of 48.1° N, 800 km of translation is indicated. Using the larger correction obtained by the E/I method reduces the translation to zero.