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Geological Society, London, Special Publications; 2004; v. 238; p. 191-203;
DOI: 10.1144/GSL.SP.2004.238.01.13
© 2004 Geological Society of London

Sedimentary Fabrics

How deformed are weakly deformed mudrocks? Insights from magnetic anisotropy

Josep M. Parés

Department of Geological Sciences, University of Michigan, 2534 C.C. Little Building, Ann Arbor, MI-48109, USA jmpares{at}umich.edu

For over thirty years the anisotropy of magnetic susceptibility (AMS) has been exploited in rock fabric studies. Our knowledge of the mechanisms leading to magnetic fabric development in deformed rocks has grown substantially, even though some details are still under debate. This paper reviews recent developments in AMS studies applied to the characterization of early deformation stages in mudrocks. From the current data set of AMS in rocks deformed at low temperature and low pressure it is possible to revisit the concept of weakly deformed rocks. We contend that weak deformation in mudrocks and the appearance of magnetic cryptofabric are concatenated. Furthermore, magnetic anisotropy studies in weakly deformed rocks suggest that cleavage fabric, the most common planar fabric in deformed mudrocks, builds up gradually from the earliest stages of deformation rather than suddenly at some strain threshold.