Discrimination between conical and cylindrical folds is crucial where conical folds influence surrounding structural geometry or when geologic data are projected into cross sections. Poles to cylindrical and circular conical folds plot on stereographic projections along great and small circles, respectively. Best-fit great and small circles to a girdle of poles from a folded surface are calculated using a least-squares minimization computer subroutine, which is from the IMSL software library and is based upon the Levenberg–Marquardt algorithm. The minimized parameter is the sum of squares of the angular separations between all poles to the folded surface and the great or small circle. A nonparametric, signed rank test of Wilcoxon is then used to statistically discriminate between cylindrical and circular conical fold models. Statistical conclusions are checked against an applicable parametric F-test.A syncline and a flanking anticline near the northern termination of the Lewis thrust are used to demonstrate the method. The syncline is statistically cylindrical whereas the flanking anticline is statistically conical with a semi-apical angle of 26°.

You do not currently have access to this article.