HR: 0800h
AN: T51A-0440    [Abstracts]
TI: Mechanical Dauphin‚ Twinning in Quartz on a Regional Scale: Texture of r and z Rhombs in Mylonites from Neutron Diffraction
AU: * Pehl, J
EM: jenpehl@eps.berkeley.edu
AF: Department of Earth and Planetary Science, University of California, Berkeley, Berkeley, CA 94720 United States
AU: Wenk, H
EM: wenk@seismo.berkeley.edu
AF: Department of Earth and Planetary Science, University of California, Berkeley, Berkeley, CA 94720 United States
AB: Preferred orientation in quartz is routinely used to determine the deformation history of rocks. Most studies use only the c-axis distribution because it has been conventionally measured with the U-stage and has been discussed in terms of slip system activities. With diffraction methods, a complete orientation distribution function (ODF) can be calculated. From the ODF, the preferred orientation of any lattice direction can be determined, including overlapping peaks such as the r and z rhombs. We have used neutron diffraction, combined with Rietveld texture analysis, to determine preferred orientation of quartz in mylonites from the Santa Rosa mylonite zone in southern California. We observe that the r and z planes do not have the same textures, with r-rhombs showing a maximum normal to the foliation and z-rhombs a minimum. These patterns are consistent over a regional scale. Simple modeling shows that the difference in texture between positive and negative rhombs can be attributed to mechanical Dauphin‚ twinning, which is imposed on an initial deformation texture produced by slip through compression. The r and z planes have different compliances and Dauphin‚ twinning acts to maximize stored elastic strain energy. Reversing the sense of twinning produces identical textures in the two rhombs. Dauphin‚ twinning therefore occurs concurrently with regional deformation and is not an expression of late-stage deformation governed by local stresses. Due to the orientation dependent nature of the twinning, it may be possible to infer stress direction from texture patterns.
DE: 8099 General or miscellaneous
DE: 5199 General or miscellaneous
DE: 3954 X ray, neutron, and electron spectroscopy and diffraction
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting