Bias correction for the orientation distribution of slump fold axes: Application to the Cretaceous Izumi basin

Linear structures perpendicular to an outcrop surface are easily discovered, but those parallel to the surface are not, giving rise to a biased orientation distribution of the structures. Here, we propose a bias correction method: Statistical inversion was conducted to unbias the distribution of the axes of mesoscale slump folds in the Cretaceous Izumi Group, Japan using the orientation distribution of outcrop surfaces. The observed axes showed a cluster in the SE quadrant. Their unbiased distribution had a girdle pattern with a maximum concentration orientation in the same quadrant, but the unbiased one had a lower peak density than the observed one, and was more girdle-like than the observed one. The maximum concentration axis of the unbiased distribution was roughly perpendicular to the paleocurrents observed in the same area. Therefore, the popular view that the axes of slump folds are perpendicular to paleoslope applies to the folds in the area in a statistical sense. The hypothesis about the vergences of slump folds and paleoslope hold only about a half of the observed slump folds.