Tectonic Geomorphology of Active Folding and Development of Transverse Drainages

Two fundamental geomorphic questions are: (1) Do growing folds propagate laterally? and (2) how does transverse drainage develop across an active fold belt? Research during the past decade has answered the first question, and it is now known that some folds in fold-and-thrust belts definitely propagate laterally. Individual folds may propagate in one or both directions, and two folds may propagate toward each other, producing a suture zone and a longer segmented structure. Folds may also propagate laterally past one another to form an en echelon arrangement. Rates of lateral propagation and incision of streams that traverse growing folds remain poorly constrained. Adequate chronology to quantify fold growth and erosion is commonly not available and therefore remains one of the greatest limitations to understanding the evolution of surface folds. As a result, structural and geomorphic analyses are utilized, which are useful, but not adequate for quantifying fold growth processes. Development of transverse drainage across folds is a complex process related to the rate of uplift, lateral, and vertical fold growth (including change in fold width), development of water and wind gaps, lateral diversion of channels, and change in channel geometry and unit stream power. Understanding lateral propagation has important implications for earthquake hazard. If fault length is assumed to increase with fault displacement, then the folds they produce should also increase in length. Hypothetically, as folds propagate laterally due to displacement on the underlying faults, coseismic displacement along the faults themselves may have a preferred direction of propagation.