The experimental and numerical deformation work discussed above focused primarily on the effects of tectonic strain on magnetic fabric and remanence. Tectonic effects usually occur relatively late in the magnetic history of the rock, long after the primary remanence and magnetic fabric have been acquired. In the past four years U.S. workers also conducted magnetic fabric-deformation studies designed to recognize and ultimately correct for the effects of an early stage deformation effect, burial compaction.
Jackson et al. [1991] carried out a pioneering theoretical and experimental study in which they used the remanence anisotropy (AAR) of a sediment or sedimentary rock to recognize and correct for inclination shallowing. In developing a theoretical relationship between AAR intensity and the magnitude of NRM inclination shallowing, they demonstrated that the magnetic anisotropy of the individual magnetic particles is a major control on magnetic fabric intensity. A series of laboratory redeposition experiments using both acicular and equidimensional, submicron-sized magnetite particles in a nonmagnetic kaolinite matrix showed the accuracy of this relationship. Collombat et al. [1993] employed a similar approach when they studied inclination shallowing in natural, recent marine sediments from the Atlantic Ocean. They found a strong correlation between anhysteretic anisotropy (AAR) and inclination shallowing which lead them to suggest that AAR can be used to recognize and correct for inclination shallowing.
One major assumption of Jackson et al.'s approach is that the magnetic particle distribution in a sediment will develop a preferred orientation in a regular manner with increasing volume loss during burial compaction. Kodama and Sun [1992] conducted laboratory compaction experiments and measured remanence and AAR magnetic fabric measurements at different stages of compaction in order to test this assumption. Kodama and Sun compacted synthetic and natural sediments which contained acicular magnetite grains in a kaolinite clay matrix. The highly anisotropic grains (length:width=5:1) should be able to check the magnetic grain anisotropy dependence of the Jackson et al. model. Kodama and Sun found a discrepancy between their results and the Jackson et al. model during early stages of compaction, at high sediment void ratios, and suggested that randomization of magnetic particles during the early development of clay fabric causes an irregular relationship between magnetic fabric development and volume loss. They suggested that compaction experiments on each rock or sediment type being studied should be conducted so that a sediment's unique fabric vs. volume loss relationship could be determined.
Sun and Kodama [1992] conducted a detailed magnetic fabric (AAR) and non-magnetic fabric (X ray pole figure goniometry, SEM) study of laboratory compacted magnetite-clay synthetic and natural marine sediments to better understand the factors controlling inclination shallowing at the microscopic scale. Sun and Kodama found evidence from SEM observations that magnetite grains stick to clay particles. By comparing AAR measurements to X ray pole figure goniometry and SEM measurements at different stages of compaction they also found that magnetic fabric development closely mimics clay fabric development after an initial randomization of magnetic particles during the early stages of clay fabric development. Once the clay fabric is established and magnetite grains are attached to clay particles or incorporated into clay domains, magnetic fabric develops in a regular way as clay particles reorient. Approaches like those developed by Jackson et al. [1991] would then be accurate models for magnetic fabric-inclination shallowing corrections. Hodych and Bijaksana [1993] and Hodych and Buchan [1994] have followed this general approach and have tried to use remanence anisotropy (AAR for magnetite containing limestones; AIR for hematite-containing redbeds) to detect and correct for compaction-caused inclination shallowing in ancient sedimentary rocks.