HR: 15:25h
AN: GP12A-08    [PDF]
TI: Rotation of Uniaxial Ellipsoidal Particles During Simple Shear Revisited: Implications for the Interpretation of AMS Measurements.
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
AU: Ch\'{a}vez-\'{A}lvarez, M J
EM: jchavez@cicese.mx
AF: CICESE, Dept. of Geology, P.O. Box 434843, San Diego, CA 92143 United States
AB: The rotation of uniaxial ellipsoidal particles has provided the base for many studies of shape preferred orientations (SPOs) of mineral grains, including those concerned with the anisotropy of magnetic susceptibility (AMS) of a large variety of rocks. In most AMS studies, it has been customary to assume that a stable orientation is achieved irrespective either of the elongation ratio of the particle, or of the amount of shear experienced by the rock. In contrast, fabric studies other than AMS commonly have made much emphasis in the cyclic movement of the particles, concluding that no stable orientation can be achieved in simple shear. In this work we studied the evolution of a multiparticle system as a function of 1) the elongation ratio of the particles, 2) the initial particle distribution and 3) the amount of shear experienced by the rock. Our model reveals that all of these factors are important in controlling the acquisition of a stable orientation upon deformation, and can be used to establish adequate threshold values depending of the conditions of interest. The more elongated particles (r = short / long semiaxis $<$ 0.2) will define a stable fabric in almost any situation of geological interest. Less elongated particles (r $>$ 0.5) can also be considered to have achieved a stable orientation if deformation remains low ($\gamma$ $<$ 3). Consequently, it is necessary to distinguish between rocks in which the SPO is suspected to have been achieved under solid deformation and those in which the SPO may have been acquired upon magmatic deformationand for which shear can be much larger ($\gamma$ $\sim$ 80). Of particular relevance for AMS studies is the finding that it is possible to observe some systems with less elongated particles in which the mean orientations are perpendicular to the direction of shear. This implies that many "abnormal" fabrics may have been the result of the natural evolution of a simple multiparticle system, and not to turbulence or post-emplacement alteration effects. Our model results also suggest that a systematic study of samples collected at various controlled positions within the same rock can provide enough information to evaluate whether the mean SPO of the minerals (and hence the associated AMS) is parallel to the direction of shear.
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