HR: 0830h
AN: GP11D-0286    [PDF]
TI: The AMS of Dykes: Should Kmax Axes be Imbricated Everywhere?
AU: * Chavez-Alvarez, M J
EM: jchavez@cicese.mx
AF: CICESE, Dept. of Geology, P.O. Box 434843, San Diego, CA 92143 United States
AU: Ca\~{n}\'{o}n-Tapia, E
EM: ecanon@cicese.mx
AF: CICESE, Dept. of Geology, P.O. Box 434843, San Diego, CA 92143 United States
AB: Most studies of the anisotropy of magnetic susceptibility (AMS) of dykes have assumed that the axes of maximum susceptibility (kmax) should define an opposed imbrication pointing along the direction of magma flow, and that this orientation should be preserved along the dyke. This assumption is supposedly based in a model predicting the orientation of ellipsoidal particles immersed in a moving liquid. However, this model actually predicts a cyclic movement of the ellipsoidal particles that has been overlooked without further justification. By using the complete model of ellipsoidal movement, we have developed a model of the expected AMS on different places within the dyke. A consequence of the cyclic movement of the particles is that the imbrication of kmax axes is not preserved along flow direction. Further, depending on the particle elongation ratio and on the amount of shear, it is possible to find an imbrication that is pointing in the wrong direction. Fortunately, by paying attention to the systematic variations of the orientation of the particles as predicted by the complete model of particle movement, it is possible to determine flow directions confidently. Moreover, our results explain satisfactorily recent observations of variations of AMS along flow direction in the same dyke, and provide simple explanations for many of the "abnormal" fabrics reported in many dyke swarms around the world.
DE: 1518 Magnetic fabrics and anisotropy
DE: 8030 Microstructures
DE: 8429 Lava rheology and morphology
DE: 8434 Magma migration
DE: 8439 Physics and chemistry of magma bodies
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2003 Fall Meeting