Modified Busk-Kink Method for Constructing Quick Geologically Admissible Cross Sections of Parallel Folds

A modified version of the classic Busk (1929) and Kink (Suppe, 1989) methods provides students with an effective way to construct geologically admissible cross sections of strata involved in parallel folds. This method is especially well suited for students participating in field methods and summer field courses, who must construct cross sections from their own geologic map data. Students in such courses are often encouraged to use their field data to sketch subsurface projections. Unfortunately, while constructing cross sections with freehand projections is quick and effective, this technique requires a level of experience and insight that most students are still developing. Instead, students often rely upon the mechanically rigorous methods (e.g., Busk/Arc and Kink methods) of constructing cross sections that they learn in structural geology courses. However, these formal methods can be cumbersome and often require specialized drafting tools. Furthermore, successfully implementing these traditional methods in field courses is often hindered by even slight inconsistencies in student field data, which can lead to frustrating complications.

Combining the mechanical control of traditional techniques with the quickness of the freehand approach, the modified method begins with a topographic profile containing geologic contacts and dip ticks, which illustrate the direction and magnitude of dipping strata. As with the Busk method, lines of constant dip (LCD) are extended perpendicular to each dip tick. To simplify complex fold shapes, crossing LCDs are supplanted with a single LCD that bisects the angle formed at their intersection. Then, once a framework of LCDs is established, geologic contacts and form lines are projected through the cross section as straight-line segments perpendicular to each LCD. As with the Kink method, it is the intersections of these straight-line projections that define fold hinges. Finally, the projections are smoothed by hand into a series of inflection points, circular-arc, and straight-line segments.