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The correlation of magnetic anisotropy with strain in the Chelmsford Formation of the Sudbury Basin, Ontario

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DOI: 10.1016/0040-1951(88)90194-1


Abstract A quantitative correlation has been established between finite strain and the anisotropy of magnetic susceptibility (AMS) in sandstones of the Precambrian Chelmsford Formation of the Sudbury Basin, Ontario. Investigations of the progressive acquisition of isothermal remanent magnetization and its destruction with reverse field and thermal demagnetization as well as microscopic examination of polished thin sections show that both pyrrhotite and magnetite are present in the sandstone. Measurements of the AMS have been carried out at 16 sites distributed throughout the Chelmsford Formation outcrop. Considerable variation in the shapes of the magnetic susceptibility ellipsoids (axes k max > k int > k min exists within each site, with the k max and k int axes contained in the cleavage planes, where developed. The magnitudes of the magnetic susceptibility axes have been quantitatively correlated with strain measurements carried out by Clendenen et al. (this issue). Two different strain to magnetics correlations have been carried out: between the site mean strain determined using deformed concretions and the mean values of the AMS at each site, and between individual specimen strain values determined at the microscopic scale and the individual specimen AMS. In both cases, separate correlations were made between strain long, intermediate and short axes and their corresponding magnetic k max , k int , and k min axes. At 10 sites, the site mean strain values were derived from strain analysis of the concretions and at an additional 6 sites the site mean strain values were derived from the AMS to strain correlation. 527 individual cores were converted to equivalent strain values using the AMS-structural correlation of individual specimens. This study extends the use of AMS methods to include strain determination in sandstones, and provides an order of magnitude increase in the available number of such data for further structural analysis.

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