HR: 14:40h
AN: GP12A-05 INVITED     [PDF]
TI: How Deformed are Weakly Deformed Rocks? Insights From Magnetic Anisotropy
AU: * Pares, J M
EM: jmpares@umich.edu
AF: Josep M PARES, Department of Geological Sciences, 2534 C.C. Little Building, University of Michigan, Ann Arbor, MI 48109
AB: Early stages of rock deformation typically involve preferred orientation of minerals or fabric elements. Magnetic anisotropy is an extremely sensitive and accurate technique that can be used to determine tectonic fabrics and thus is an ideal approach to characterize weakly deformed rocks. In mudrocks, Anisotropy of Magnetic Susceptibility resides mostly on phyllosilicate grains and the degree of preferred orientation can then be taken as a deformation intensity gauge. A number of field-based and experimental studies show that under very low strain and incipient re-orientation, phyllosilicate platy grains interact with each other to produce a rather common magnetic ellipsoid where the principal maximum axes of susceptibility lie parallel to the bedding and flattening plane intersection. Under progressive incremental strain, principal axes of maximum susceptibility eventually parallel the tectonic stretching direction, depending on the strain and both magnitude and orientation of the original (pre-deformational) fabric. This type of early fabrics is very often characterized by prolate magnetic ellipsoids with relatively low values of anisotropy degree (P). The magnetic ellipsoid evolution under increasing strain shows an initial decrease in anisotropy degree followed by an almost exponential growth. The magnetic shape factor (T) changes from oblate (primary-sedimentary fabric) to prolate and back to the oblate field for higher strains. The initial cryptofabric (prolate ellipsoid, low value of P) occurs usually before the appearance of discernible cleavage and thus indicates the first evidence for penetrative tectonic fabrics in rocks. There is an overwhelming number of studies, mostly from the Alpine realm, revealing magnetic cryptofabric in mudrocks otherwise lacking evidence for tectonic deformation (such as pressure-solution, pencil structures, cleavage). Due to the high sensitivity of magnetic anisotropy methods, we are pushing the frontiers of what is classically been called weakly deformed rocks. For example, the question of whether the appearance of cleavage in mudrocks occurs at some strain threshold or instead it builds up gradually from the earliest stages of deformation and has simply escaped noticed, has been debated for many years. Our recent advances in magnetic anisotropy of "weakly deformed rocks" not only favor the later, but also support that magnetic cryptofabric, which is typically linear, is the precursor of what we have been calling incipient cleavage in mudrocks.
DE: 1518 Magnetic fabrics and anisotropy
DE: 5109 Magnetic and electrical properties
DE: 8025 Mesoscopic fabrics
DE: 8030 Microstructures
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2003 Fall Meeting