Investigating topographic metrics to decipher structural model and morphotectonic evolution of the Frontal Siwalik Ranges, Central Himalaya, Nepal

Abstract

The fold-and-thrust belt in the Himalayan front is marked by the upliftment of the Frontal/Outermost Siwalik Hills along the Main Frontal Thrust (MFT). These frontal ranges display distinct morphology with prominent curved/stair-stepped and overlapping segments in the central Himalaya. In this study, we analyse the structural framework, morphology, growth and relative uplift rates of the frontal ranges in the central Himalaya, using various topographic metrics. The results suggest that the frontal ranges are folds developed along six MFT segments that have interacted to form the present day topography. These segments are also overlapping in some areas, thus, resulting in stress partitioning. We also identify a NW-SE trending pre-existing tear fault in the region that forms barrier to the growth and propagation of the MFT segments and the frontal ranges along it. The uplift rates are generally lower on relatively older fault segments due to initiation of the younger overlapping faults. It is supported by the results of the morphometric indices as the older and less active MFT segments yield high mountain front sinuosity ratio (1.3–2.2) and valley floor width to height ratio (3.3–14.6), and low values of steepness index (0.2–0.8) and hypsometric integral (0.4–0.5). Growing folds along the MFT segments have resulted in the deflection of the Narayani and the East Rapti rivers. We establish a model for the initiation and growth of various structural segments in the study area and the resultant drainage reorganisation.