Physics
Scientific paper
Aug 1988
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1988pepi...51..290g&link_type=abstract
Physics of the Earth and Planetary Interiors, Volume 51, Issue 4, p. 290-300.
Physics
11
Scientific paper
This study presents the results of magnetic susceptibility anistropy measurements on mylonites from the Brevard Zone of the southern Appalachians. The Brevard Zone is a major crustal mylonite zone and the samples analyzed for this study represent the progressive increase of mylonitization (decrease in grain size) of the Henderson Gneiss with increasing proximity to the Brevard Zone. Our results suggest that magnetic fabric measurements in mylonites are at least qualitatively significant with respect to the strain history of mylonites. The orientations of the principal susceptibilities are parallel to rock fabric elements indicating that the magnetic fabric is a deformationally induced phenomenon. Although the Henderson Gneiss appears to undergo a smooth progressive mylonitization with increasing proximity to the Brevard fault zone, our magnetic measurements suggest that strain states do not increase smoothly as the Brevard Zone is approached. However, the magnetic measurements do suggest that at some sites the strain is very high. The highest site mean anisotropy [(max-min)/mean × 100] we have measured is 120% (cgs) which could, based on strain to magnetic susceptibility anisotropy (MSA) correlations by other workers, indicate strain ratios (1 +e1)/(1 +e3) of between 186 and 1.4 × 1017. Because we cannot correlate our magnetic fabric measurements with strain in these rocks, we cannot prove that such high strain states in fact exist in the Brevard mylonites. However, these appear to the highest magnetic anisotropies yet recorded. Our data also indicate that the susceptibility ellipsoid is severely flattened with k values commonly lower than than 0.1. We also observe an inverse relationship between k values and anistropy. Similar relationships have been observed in other shear zones and suggest that mylonites follow a strain path of increasing departure from plane strain (k = 1).
Brown Laurie L.
Goldstein Arthur G.
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