Magnetic anisotropy as a correction for compaction-caused palaeomagnetic inclination shallowing

Abstract

A theoretical model presented by Jackson et al. (1991) suggests that the anisotropy of anhysteretic remanence (AAR) can be used to recognize and correct for compaction-caused inclination shallowing in sediments and sedimentary rocks. However, we have conducted experiments in which inclination shallowing is more strongly dependent on magnetic anisotropy during compaction of synthetic, clay-rich sediments containing 0.45 μm acicular magnetite than predicted by the Jackson etal. model. This discrepancy can be explained if randomization or disturbance of subvertical magnetite particles is the cause of inclination shallowing early in the compaction process. The principal experimental evidence for this effect is a decrease in magnetization intensity coincident with inclination shallowing. An intensity decrease during inclination shallowing is also observed in some DSDP cores suggesting that this randomization process may also occur in nature. After randomization magnetite particles become firmly attached to clay particles and both magnetic anisotropy and inclination shallowing show a regular increase during compaction consistent with the Jackson etal. model. We suggest that models like that of Jackson etal. can be used to correct for inclination shallowing if they are used in conjunction with an experimentally-derived relationship between inclination shallowing and magnetic anisotropy for a particular sediment.