Deciphering a Duplex: an Example From the Western Nepal Himalaya

Because no unique answer exists for the restoration of a cross section, critical decisions must be made during the restoration process which determine the amount of shortening and overall character of the cross section. Often these decisions are straight-forward. Yet, inevitably, assumptions must be made to continue with the restoration. It is especially difficult when data density is minimal, as is true in Nepal. The far western part of Nepal as little infrastructure and acquiring ground truth data is difficult and time consuming. Based on data from field work in the Chainpur area of the Bajhang district of far western Nepal in the Himalayan thrust belt, four models were developed for the emplacement of a duplex. Uncertainty resided in the number of faults, whether some thrust sheets were folded, and location of faults in areas that were inaccessible. Ultimately, one model was chosen based on its structural probability, fit to the ground truth data around Chainpur, and fit into the overall kinematic development of the thrust belt. A cross section was developed for the Himalayan thrust belt in far western Nepal that bisected the Chainpur area. Based on the model described above, two kinematic restorations were possible. Although the first reconstruction required more shortening, the second reconstruction utilized kinematically creative faults. I used a forward modeling approach to help understand the kinematic viability of the restorations. By starting the reconstruction at approximately 25 Ma with the emplacement of the Main Central thrust sheet and working forward in time, the reconstruction program, 2-d Move, determined that the first reconstruction was viable but the second reconstruction was kinematically impossible. This decision led to an increase in the amount of shortening in the cross section. Just as important, this example illustrates the utility of using forward modeling for the reconstruction of orogen-scale cross sections.