We have studied the magnetic fabric of Paleozoic carbonates from numerous sites in the North American midcontinent, for comparison with paleomagnetic behavior and calcite twinning strains. Sites carrying a Kiaman remagnetization have significantly lower anhysteretic remanent magnetization (ARM) anisotropy (less than 3%) than other sites. Minimum axes of ARM ellipsoids are distributed in a vertical plane trending approximately SE-NW. This trend is very similar to the orientation of Alleghenian layer-parallel shortening, as inferred by mechanical twinning in calcite. Sites lacking a Kiaman overprint invariably have steeply-oriented minimum ARM axes and relatively strong anisotropy, whereas the most strongly remagnetized sites have nearly horizontal minima and weak anisotropy. These observations suggest that diagenetic/authigenic magnetites formed independently of any pre-existing fabric of precursor sulphides and host rock, and that the magnetic fabric of the whole midcontinent was at least partially affected by Alleghenian syn-orogenic activity.
Bibliographical noteFunding Information:
We thank StephenM arshak and Graham Bor-radaile for helpful reviews and comments.S up-portedb y the National ScienceF oundationG rant EAR 90-05075a nd EAR 9044181.I nstitute for Rock Magnetismc ontribution #9204.T he IRM is funded by the Keck Foundation, the National Science Foundation, and the University of Minnesota.
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