The fold test in paleomagnetism as a parameter estimation problem

Abstract

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.

New Early Permian paleomagnetic results from the Brive basin (French Massif Central) and their implications for Late Variscan tectonics

“…6a, b) confidence levels, respectively). Proportional incremental unfolding according to Watson and Enkin (1993) confirms this conclusion with a maximum k value at 98.6% unfolding (Fig. 7).…”

Remagnetization of Upper Jurassic limestones from the Danubian Unit (Southern Carpathians, Romania): tectonic implications

“…In four localities (B4, B5, B8A, B8B) we sampled both limbs of a mesoscopic fold so it was possible to do local fold tests. We used two types of fold test: proportional fold test (Watson & Enkin 1993) and direction-correction tilt test (Enkin 2003). Proportional Fold Test evaluates the unfolding % which corresponds to the maximum Fisher precision parameter k as the data is progressively unfolded.…”

Middle Eocene paleomagnetic data from the eastern Sakarya Zone and the central Pontides: Implications for the tectonic evolution of north central Anatolia

“…[41] Paleomagnetic fold tests associated with progressive unfolding [Watson and Enkin, 1993] were applied both to the individual regional groups of this study and to the whole data set available from previous studies. Almost all paleomagnetic groups in NCA are carriers of a prefolding remanent magnetization, although the means of some groups cluster best after untilting at 60%, suggesting a syntectonic deformation.…”

“…[32] Because the lavas are flat lying over most of the study area, only a restricted number of sites with tectonic tilt correction could be used to perform a fold test following the method of Watson and Enkin [1993] (see Appendix B). Some neighboring groups were joined together because of low tilts.…”

“…[58] The fold test after Watson and Enkin [1993] has been performed on the different groups in Table 2 to constrain the age of magnetization. The optimal unfolding between 80.2% and 109.1% for groups G2+G3, G5, and G8−10 shows that folding occurs before deformation, whereas G4 and G6 were not considered because of insufficient tilt corrected data (see Table B1).…”