Magnetic signatures of rock deformation

Magnetic properties of rocks are pressure-sensitive and frequently used for revealing the dynamics of tectonic-related deformation phenomena within the Earth’s crust [1]. Among different magnetic methods, the anisotropy of magnetic susceptibility (AMS) is a well-established method in petrofabric analyses. In metamorphic rocks, AMS can be pronounced and magnetic lineations and foliations are the result of plastic deformation under directional static stress [2]. A serious modification of magnetic properties occurs during hypervelocity impacts. Magnetic and microstructural studies have shown that e.g. a decrease in magnetic susceptibility is mainly related to a shock-induced refinement of magnetic domains in a cooperative process of brittle and ductile deformation mechanisms [3]. Up to now, modification of magnetic properties are mainly reported from static or dynamic shock-related deformation experiments. However, it is generally accepted that tectonic stress accumulates non-uniformly, but remains quite low until a rapid stress variation occurs shortly before and after earthquake propagation or volcano eruption. Rapid stress increase may be especially favorable if tectonic loading exhibits a cyclic or fatigue character. ... mehrKnowledge on the effect of cyclic, frequency-dependent loading on the magnetic behavior (magnetic fatigue) of rocks is poor up to now. Therefore, we carried out cyclic uniaxial compression experiments using an universal testing Instron machine and a GABO Eplexor Dynamic Mechanical Analysis (DMA) system. Modifications in magnetic properties were studied by measuring the low-field magnetic susceptibility and the Verwey transition temperature of magnetite. Again, a cooperative interplay of brittle and ductile deformation mechanisms can be observed explaining the observed magnetic fatigue effect. We expect from this project basic knowledge on the piezomagnetic stress sensitivity of crustal rocks, which is currently poorly constrained. The outcome might be applied in geosciences to understand the influence of deformation on magnetic signals in tectonically active areas and help to interpret their temporal variations.

References

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