Flow directions in dikes from anisotropy of magnetic susceptibility data: The bootstrap way

Tauxe, L, Gee JS, Staudigel H.  1998.  Flow directions in dikes from anisotropy of magnetic susceptibility data: The bootstrap way. Journal of Geophysical Research-Solid Earth. 103:17775-17790.

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magma, rocks, strain


One of the first applications of anisotropy of magnetic susceptibility (AMS) was an attempt to determine flow directions from mafic dikes [Khan, 1962]. Since the seminal work of Knight and Walker [1988] defining the expected behavior of AMS in response to magma flow, there has been increasing interest in using AMS for this purpose. Here we present a quantitative method for interpretation of AMS data from dikes, using a parametric bootstrap. First, dikes must be sampled with at least five land preferrably more) samples from within 10 cm of the dike margin. The distributions of the eigenvalues and eigenvectors of the AMS tensor are delineated by calculating eigenparameters of many bootstrapped paradata sets. We generate paradata sets by first selecting a sample at random, then calculating a replacement set of data by drawing tensor elements from normal distributions with the mean and standard deviation of the entire site. The bounds containing 95% of the eigenparameters of the bootstrapped data serve as confidence limits for the parameter of interest. Classification of dikes proceeds as follows: Sites whose maximum and intermediate eigenvalues could not be distinguished are deemed uninterpretable. In addition, sites with principal eigenvectors with angles > 45 degrees away from the dike margin (inverse) or with markedly different directions on either side of the dike (scissored) are excluded. The remaining dikes are classified as having unique flow direction information if the principal eigenvectors from at least one side are distinct from the dike plane based on the distribution of the bootstrapped principal eigenvectors. If neither side has principal eigenvectors distinct from the dike plane, the dikes are classified as having lineation information only. A study comprising 251 dikes from the Troodos ophiolite has 151 sites with directional data, 38 sites with lineations only, 7 inverse sites, 5 scissored sites, and 55 sites not fitting into any other category. The flow directions interpreted from the data were generally southerly, toward a fossil transform zone.