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| Titel |
Multifractal model of magnetic susceptibility distributions in some igneous rocks |
| VerfasserIn |
M. E. Gettings |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1023-5809
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| Digitales Dokument |
URL |
| Erschienen |
In: Nonlinear Processes in Geophysics ; 19, no. 6 ; Nr. 19, no. 6 (2012-11-23), S.635-642 |
| Datensatznummer |
250014262
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| Publikation (Nr.) |
 copernicus.org/npg-19-635-2012.pdf |
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| Zusammenfassung |
| Measurements of in-situ magnetic susceptibility were compiled from mainly
Precambrian crystalline basement rocks beneath the Colorado Plateau and
ranges in Arizona, Colorado, and New Mexico. The susceptibility meter used
measures about 30 cm3 of rock and measures variations in the modal
distribution of magnetic minerals that form a minor component volumetrically
in these coarsely crystalline granitic to granodioritic rocks. Recent
measurements include 50–150 measurements on each outcrop, and show that the
distribution of magnetic susceptibilities is highly variable, multimodal and
strongly non-Gaussian. Although the distribution of magnetic susceptibility
is well known to be multifractal, the small number of data points at an
outcrop precludes calculation of the multifractal spectrum by conventional
methods. Instead, a brute force approach was adopted using multiplicative
cascade models to fit the outcrop scale variability of magnetic minerals.
Model segment proportion and length parameters resulted in 26 676 models to
span parameter space. Distributions at each outcrop were normalized to unity
magnetic susceptibility and added to compare all data for a rock body
accounting for variations in petrology and alteration. Once the best-fitting
model was found, the equation relating the segment proportion and length
parameters was solved numerically to yield the multifractal spectrum
estimate. For the best fits, the relative density (the proportion divided by
the segment length) of one segment tends to be dominant and the other two
densities are smaller and nearly equal. No other consistent relationships
between the best fit parameters were identified. The multifractal spectrum
estimates appear to distinguish between metamorphic gneiss sites and sites on
plutons, even if the plutons have been metamorphosed. In particular, rocks
that have undergone multiple tectonic events tend to have a larger range of
scaling exponents. |
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