ABSTRACT

While it is common knowledge that fold plunge may introduce error in the geometrical restoration of current indicators if not properly considered, the various permutations of possible errors have not been detailed from actual field cases. The necessity to consider plunge very carefully in order to obtain reliable paleocurrent results from flysch strata involved in asymmetrical buckle folds and possessing an unusually penetrative cleavage on South Georgia Island revealed many of those permutations. In practically all cases--be it for cross sets or sole marks--restorations that considered plunge showed significantly smaller dispersions, and in many cases the mean-vector orientation was shifted as much as 90°. In a few cases, clear bi-modal patterns emerged, which were not apparent f om simple strike rotations.

For tightly folded sequences, it is always best to rotate around a fold hinge or bedding-cleavage intersection. Moreover, it is preferable to do such restorations by hand while in the field so that one can quickly compare the paleocurrent and structural patterns and minimize possible errors. On South Georgia, for example, it would be easy to amass a large body of meaningless data from pseudo-sole marks were one not alert at all times to the orientations of bedding-cleavage intersections. In similar folds where there has been much internal flow, restoration becomes extremely difficult, and only if the apparent resultant current direction is nearly perpendicular to the maximum regional structural extension direction can the results be trusted.