Magnetic fabric and inclination shallowing studies: depositional and post-depositional processes in hematite- and magnetite-bearing rocks

Abstract

Magnetic anisotropy-based inclinations corrections of both hematite and magnetite-bearing sedimentary rocks indicate latitudinal variations of inclination shallowing. Rocks formed at mid latitudes suffer from more shallowing than those formed closer to the equator, consistent with the tan Im= f * tan If relationship, where Im is the measured inclination and If is the field inclination during deposition. Shallowing of the paleomagnetic vectors can be expressed in terms of the flattening factor f, a function of the rock’s magnetic fabric and the individual particle anisotropy, the a factor. Estimation of the f factor enables performing simplified inclination corrections. f factors derived from anisotropy-based inclination corrections were combined with f factors derived from corrections that use models of geomagnetic field secular variation for hematite and magnetite bearing rocks. Magnetite data indicate a smaller range of f factors, leading to smaller ranges of inclination shallowing. Using the reported range of f factors enables more precise estimations of inclination corrections. Hematite data, on the other hand, show a broader range of f factors, which makes estimating inclination shallowing and correcting for it more difficult. However, because hematite has magneto-crystalline anisotropy, the value of a doesn’t have much variation, thus requiring a precise measure of the magnetic fabric only. Hematite fabrics were measured for an inclination shallowing study of red beds from the Maritime Provinces of Canada (Shepody Fm), using a high field anisotropy of isothermal remanence technique (hf-AIR). The technique allows to fully saturate hematite’s remanence without the need to demagnetize the samples between the different positions required to measure the anisotropy tensor, eliminating the risk of thermo-chemical alteration. The technique makes it possible for typical paleomagnetic laboratories to measure the remanence anisotropy of high coercivity hematite. The precise measurement of the fabric allowed interpreting it as a compactional fabric that was reoriented by strain during folding following a flexural-slip model.