Partial Anhysteretic Anisotropy Measured in the Greys Landing Ignimbrite of the Central Snake River Plain

Abstract:

Magnetic remanence directions recorded in the glassy sub-lithologies of mid-Miocene rheomorphic Snake River Plain ignimbrites are often discrepant compared to the more reliable directions in crystalline centers and underlying baked paleosols. The rocks have undergone no tectonic strain, and the rheomorphic deformation preserved in the rock occurs at ˜800°C, above magnetic blocking temperatures. Accounting for the discrepantly shallow directions is critical for the use of magnetic remanence for stratigraphic correlation and structural/tectonic reconstructions. Here we present paleomagnetic and rock magnetic data from the Grey’s Landing Ignimbrite that demonstrate a strong magnetic anisotropy carried by pseudo-single to single domain magnetite grains which deflect the remanence direction by up to 40°. Strongly lineated anisotropic samples collected at distant sections ( ˜20 km separation) have their remanence deflected toward the respective flow directions inferred from their directions of maximum magnetic susceptibility (K1). Shallow K1 directions in the basal vitrophyre cause a shallowing of magnetic remanence, while a range of steep to shallow K1 directions in the folded upper vitrophyre cause both a steepening and shallowing of the remanence, respectively. There is a strong relationship between the magnitudes of remanence deflection, anisotropy of thermal remanence, coercivity, and strength of natural remanent magnetization between individual samples. There is also a strong relationship between the magnitudes of partial anisotropy of anhysteretic remanent magnetization (pAARM) and the deflection of the remanence vector difference directions, which both increase significantly with higher alternating magnetic fields. Correction of the vector difference direction using the inverse of the pAARM tensor for the same AF range is moderately successful. Previous work suggests that curvilinear demagnetization trends in the basal vitrophyre of an ignimbrite were evidence of grain rotation below the unblocking temperature. This work, however, shows that this observation is sometimes caused by a magnetic anisotropy that is increasingly larger in grains with higher coercivity (and probably also unblocking temperature).