Paleomagnetism of the Permian Wallowa terrane: Implications for terrane migration and orogeny

Abstract

A sequence of volcaniclastic sedimentary and pyroclastic rocks of Permian age (late Leonardian or early Guadalupian; ≈260 Ma) in the Wallowa terrane of Oregon and Idaho has been sampled for paleomagnetic study. Forty‐six sites (222 cores) were collected from folded, laminated siltstone, mudstone, and tuffaceous sandstone beds exposed along the Snake River in Hells Canyon. Several stratigraphic sections, within the 1500‐ to 2500‐m‐thick Hunsaker Creek Formation, were sampled over a stratigraphic interval of about 700 m. Thermal demagnetization revealed three components of magnetization. The A component has the lowest unblocking temperatures (generally less than about 200°C), has normal polarity, and fails the fold test. The B component is removed between heating steps of about 200°C and 550°C, is of normal polarity, and also fails the fold test. The C component, observed between heating steps of approximately 550°C and 660°C, is always of negative inclination. The grouping of this component improves slightly upon unfolding, with the optimum grouping occurring at 95% unfolding, using sites with n ≥ 3 and k ≥ 10, (k95/k0 = 1.18), or 100% using sites n ≥ 3 (k95/k0 = 1.51). This component is interpreted as the characteristic remanent magnetization acquired during deposition of the Hunsaker Creek Formation based on the following observations: The better grouping of the high‐temperature magnetic component after unfolding and the consistent reversed polarity seen in samples from the different sections. The resulting paleolatitude, calculated using inclination‐only statistics, of 24°±12° N (n ≥ 3, k ≥ 10), or 26°±9°N (n ≥ 3 sites), implies a northern hemisphere position of the Wallowa terrane during the Early Permian and possible southward transport of this terrane with respect to North America. If the Wallowa terrane was attached to the Wrangellia superterrane, our result indicates a northern hemisphere location for that superterrane during the Permian.