Out-of-sequence reactivation of the Munsiari thrust in the Relli River basin, Darjiling Himalaya, India: Insights from Shuttle Radar Topography Mission digital elevation model-based geomorphic indices

Abstract Quantitative tectonic geomorphology has emerged as a powerful discipline for studying evolution of topography, landscapes, and neotectonics using geomorphic indices computed from digital elevation data such as the Shuttle Radar Topography Mission (SRTM) data. We computed SRTM-based geomorphic indices to study neotectonics in the Relli River basin in the Darjiling Himalaya. We also used Real Time Kinematic Global Positioning System (RTKGPS) independent checkpoints to assess the quality of the SRTM data used to compute the geomorphic indices along with their uncertainties. Our analysis revealed that though the SRTM C-Band 90-m resolution (C90) digital elevation data has been used extensively for geomorphic studies, the 30-m resolution (C30) data were significantly more accurate. Moreover, geomorphic indices computed using SRTM C30 and C90 elevations in the Relli basin indicate that normalized, nondimensional indices such as the relief ratio ( Rh ), hypsometric integral ( HI ), basin elongation ( Re ), and valley floor width-to-height ratio ( Vf ) are statistically indistinguishable with uncertainty (1σ) at least an order of magnitude below the index value. The geomorphic indices in the Relli basin reveal neotectonic activity in the Munsiari thrust (MT) fault zone and intraformational faults in its footwall in the Lesser Himalayan rocks and also indicate that the basin is at an early mature stage close to equilibrium between tectonic and erosional process. However, analysis of the uncertainties associated with the indices suggest that the normalized or nondimensional geomorphic indices have the lowest uncertainties and that neotectonics in the Relli basin may only be confined to reactivation in the MT. The reactivation in the MT fault zone by out-of-sequence neotectonics implies the possibility of large earthquake events in the Darjiling Himalaya and significant seismic and landslide hazard for populations in large towns specifically located on the MT. Our new approach of looking at geomorphic indices and their uncertainties delivers a novel perspective for improved understanding of out-of-sequence neotectonics in river basins that may be applied more broadly across the Himalaya and elsewhere.