DEFORMATION AND PALEOMAGNETISM


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Abstract

We may use tectonic structures to confirm the primary age of a paleomagnetic remanence component but only if we know how to undo the natural strain history. It is normally insufficient to untilt fold limbs, as in the original version of Graham's Fold Test. One may need to remove also the bulk or local strain and account for strain heterogeneities, achieved by grain-strain and the more elusive intergranular flow. Most important, one must know the sequence of strains and tilts that occurred through geological history because the order of these noncommutative events critically affects the final orientation of the remanence component.In many non-metamorphic rocks, strain-rotation of a remanence component approximates a simple formula, although the actual rotation mechanism is complex. This simple, passive line approximation is confirmed experimentally for strains up to 45% oblate shortening. The passive line hypothesis has permitted successful paleomagnetic restorations in several natural case studies.Experimental deformation of samples with multicomponent remanences shows that differential stresses above a threshold value near 25 MPa selectively remove components with coercivities <25mT, due to domain wall rearrangements in large multidomain magnetite grains. Higher coercivity components are less reduced so that the net remanence vector spins always toward the high-coercivity component, at rates and along paths not predicted by any structural geological formula. Experimentally deformed samples with very fine hematite in the matrix showed their net remanence spinning away from the high coercivity component. This is due to easier mechanical disorientation of the very fine hematite grains, scattering their magnetic moments more and reducing their contribution to the overall remanence. Thus, muticomponent remanences have their components selected for survival based on rock-magnetic and microstructural criteria. Such stress-rotation by coercivity selection does not depend on the orientations of the principal stresses or strains, a concept that is counterintuitive to conventional structural geology.Syn-tectonic remagnetization is common in deformed sedimentary sequences and laboratory experiments reveal that a only moderate differential stress remagnetization is required to add components parallel to the ambient field, without significant strain. Alternating field demagnetization isolates components smeared along the great circle between the initial remanence direction and the remagnetizing field direction. In this case, the principal directions of the stress and finite strain tensors are irrelevant; remagnetization is triggered by a threshold differential stress. The final remanence direction is controlled by the ambient field direction and the remagnetization path lies along a great circle between the ambient field and the initial remanence direction.

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