SUMMARY

Previous techniques for judging the significance of a palaeomagnetic fold test are either invalid or insufficiently flexible. It is shown that under appropriate circumstances an isolated-observation test may be added to the range of statistical tests used to judge a fold test. A powerful new test is developed based on a test statistic that is sensitive to correlation between the distribution of site-mean directions about the overall mean direction and the tectonic corrections. This test is sufficiently flexible that it should cover most circumstances.

/pdf/a-new-fold-test-for-palaeomagnetic-studies-19xtpvrib7.pdf

A new fold test for palaeomagnetic studies

Configuration of the Late Paleoproterozoic Supercontinent Columbia: Insights from radiating mafic dyke swarms

Most proposed fold test formulations use significance tests to try pre-tilting and post-tilting remanence hypotheses. We suggest that it is better to consider the fold test as a parameter estimation problem. Making the usual assumption that the distribution of remanence vectors was originally roughly parallel, we propose, using a monte carlo simulation technique, to estimate the amount of tectonic tilting at the time of magnetization along with a 95% confidence interval. If, for example, this confidence interval included 100% then one could not rule out pre-tilting remanence. In the older terminology, the fold test is positive. The k-ratio test of McElhinny [1964] is often said to be conservative in that if a study passes the k-ratio test then it certainly passes a more rigorous test. We show with a typical counter-example that this assertion is incorrect. Observational uncertainty of bedding directions is easily included in this formulation.

The fold test in paleomagnetism as a parameter estimation problem

The Kalahari Craton during the assembly and dispersal of Rodinia

The fold test; an eigen analysis approach

Summary

The fold test suggested by McElhinny is shown to be inappropriate. More correctly one should determine whether the mean direction of a group of sites from one limb of a fold may be distinguished statistically from the mean direction of a group of sites from another limb. Details of the application of this test are given with examples. It is shown that the test indicates significance under far less severe folding than does the test suggested by McElhinny.

/pdf/the-fold-test-in-palaeomagnetism-3vfssvg2gt.pdf

The fold test in palaeomagnetism

SUMMARY

Earlier palaeomagnetic work on the Mashonaland dolerite sills and dykes (1830±230 Ma) showed that they record a geomagnetic field reversal. 28 new sites have been sampled in sills (26) and dykes (2) in north-east Zimbabwe. Detailed alternating-field demagnetization (AFD) is more effective in removing secondary components of remanence than thermal demagnetization (TD) and enables an assumed primary remanence to be isolated at 16 sill sites. Combined with earlier results there are 11 sill sites with a positive inclination (P) (D= 287°, I = 53°, k= 40) and 18 with a negative inclination (N)(D= 114°, I =−47°, k= 35). The data lead to a positive class-B reversal test. Field tests suggest that the N remanence is the younger and that the period of Mashonaland igneous activity spanned a reversal of the Earth's magnetic field from P to N. The areal distribution of polarity is not random: sills with only P remanence are found in the more elevated central region of the Mashonaland Igneous Province, whilst predominantly N-bearing sills occupy the lower surrounding terrain.

/pdf/a-palaeomagnetic-investigation-of-the-mashonaland-dolerites-4ok9c2bnnv.pdf

A palaeomagnetic investigation of the Mashonaland dolerites, north-east Zimbabwe

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The fold test in palaeomagnetism

A palaeomagnetic investigation of the Mashonaland dolerites, north-east Zimbabwe